Restore compatibility with Java 11 (#485)

Addresses variant resolution error demonstrated in
[eBay/metrics-for-develocity-plugin#77][1] ([job][2]).

Java 17 was introduced as target in upgrade to Gradle 9 (#454).

```
> Could not resolve all files for configuration ':compileClasspath'.
   > Could not resolve com.gabrielfeo:develocity-api-kotlin:2025.1.0.
     Required by:
         root project :
      > No matching variant of com.gabrielfeo:develocity-api-kotlin:2025.1.0 was found. The consumer was configured to find a library for use during compile-time, compatible with Java 11, preferably in the form of class files, preferably optimized for standard JVMs, and its dependencies declared externally, as well as attribute 'org.gradle.plugin.api-version' with value '8.11.1', attribute 'org.jetbrains.kotlin.platform.type' with value 'jvm' but:
Configuration cache entry discarded due to serialization error.
          - Variant 'apiElements' declares a library for use during compile-time, packaged as a jar, preferably optimized for standard JVMs, and its dependencies declared externally, as well as attribute 'org.jetbrains.kotlin.platform.type' with value 'jvm':
              - Incompatible because this component declares a component, compatible with Java 17 and the consumer needed a component, compatible with Java 11
              - Other compatible attribute:
                  - Doesn't say anything about org.gradle.plugin.api-version (required '8.11.1')
          - Variant 'javadocElements' declares a component for use during runtime, and its dependencies declared externally:
              - Incompatible because this component declares documentation and the consumer needed a library
              - Other compatible attributes:
                  - Doesn't say anything about its elements (required them preferably in the form of class files)
                  - Doesn't say anything about its target Java environment (preferred optimized for standard JVMs)
                  - Doesn't say anything about its target Java version (required compatibility with Java 11)
                  - Doesn't say anything about org.gradle.plugin.api-version (required '8.11.1')
                  - Doesn't say anything about org.jetbrains.kotlin.platform.type (required 'jvm')
          - Variant 'runtimeElements' declares a library for use during runtime, packaged as a jar, preferably optimized for standard JVMs, and its dependencies declared externally, as well as attribute 'org.jetbrains.kotlin.platform.type' with value 'jvm':
              - Incompatible because this component declares a component, compatible with Java 17 and the consumer needed a component, compatible with Java 11
              - Other compatible attribute:
                  - Doesn't say anything about org.gradle.plugin.api-version (required '8.11.1')
          - Variant 'sourcesElements' declares a component for use during runtime, and its dependencies declared externally:
              - Incompatible because this component declares documentation and the consumer needed a library
              - Other compatible attributes:
                  - Doesn't say anything about its elements (required them preferably in the form of class files)
                  - Doesn't say anything about its target Java environment (preferred optimized for standard JVMs)
                  - Doesn't say anything about its target Java version (required compatibility with Java 11)
                  - Doesn't say anything about org.gradle.plugin.api-version (required '8.11.1')
                  - Doesn't say anything about org.jetbrains.kotlin.platform.type (required 'jvm')
```

[1]: eBay/metrics-for-develocity-plugin#77
[2]:
https://github.com/eBay/metrics-for-develocity-plugin/actions/runs/17635723239/job/50111470306?pr=77

Author: gabrielfeo

library/build.gradle.kts

@@ -1,3 +1,4 @@
+import org.jetbrains.kotlin.gradle.dsl.JvmTarget
 import org.jetbrains.kotlin.gradle.dsl.KotlinVersion
 import org.jetbrains.kotlin.gradle.tasks.KotlinCompile
 
@@ -100,9 +101,15 @@ publishing {
     }
 }
 
+tasks.named("compileJava", JavaCompile::class) {
+    sourceCompatibility = JavaVersion.VERSION_11.majorVersion
+    targetCompatibility = JavaVersion.VERSION_11.majorVersion
+}
+
 tasks.named("compileKotlin", KotlinCompile::class) {
     compilerOptions {
         languageVersion = KotlinVersion.KOTLIN_1_8
+        jvmTarget = JvmTarget.JVM_11
     }
 }
 

library/src/examplesTest/kotlin/com/gabrielfeo/develocity/api/example/gradle/ExampleGradleTaskTest.kt

@@ -1,6 +1,6 @@
 package com.gabrielfeo.develocity.api.example.gradle
 
-import com.gabrielfeo.develocity.api.example.BuildStartTime
+import org.junit.jupiter.api.Assertions.assertFalse
 import org.junit.jupiter.api.Assertions.assertTrue
 import org.junit.jupiter.api.BeforeEach
 import org.junit.jupiter.api.MethodOrderer.OrderAnnotation
@@ -9,10 +9,10 @@ import org.junit.jupiter.api.TestMethodOrder
 import org.junit.jupiter.api.Test
 import org.junit.jupiter.api.io.TempDir
 import java.nio.file.Path
+import com.gabrielfeo.develocity.api.example.BuildStartTime
 import com.gabrielfeo.develocity.api.example.copyFromResources
 import com.gabrielfeo.develocity.api.example.runInShell
 import kotlin.io.path.div
-import java.nio.file.Files
 
 @TestMethodOrder(OrderAnnotation::class)
 class ExampleGradleTaskTest {
@@ -23,13 +23,11 @@ class ExampleGradleTaskTest {
     private val projectDir
         get() = tempDir / "examples/example-gradle-task"
 
-    private val initScriptPath
-        get() = tempDir / ResourceInitScripts.FORCE_SNAPSHOT_LIBRARY
-
     @BeforeEach
     fun setup() {
         copyFromResources("/examples", tempDir)
         copyFromResources("/${ResourceInitScripts.FORCE_SNAPSHOT_LIBRARY}", tempDir)
+        copyFromResources("/${ResourceInitScripts.REQUIRE_JAVA_11_COMPATIBILITY}", tempDir)
     }
 
     @Test
@@ -50,11 +48,18 @@ class ExampleGradleTaskTest {
         assertPerformanceMetricsOutput(output, user = "runner", period = BuildStartTime.RECENT)
     }
 
+    @Test
+    fun testJavaVersionCompatibility() {
+        val initScript = tempDir / ResourceInitScripts.REQUIRE_JAVA_11_COMPATIBILITY
+        val output = runBuild("-p buildSrc :generateExternalPluginSpecBuilders -I '$initScript'").stdout
+        assertFalse(Regex("""FAILED|Could not resolve|No matching variant""").containsMatchIn(output))
+    }
+
     private fun runBuild(gradleArgs: String) =
         runInShell(
             projectDir,
             "./gradlew --stacktrace --no-daemon",
-            "-I $initScriptPath",
+            "-I ${tempDir / ResourceInitScripts.FORCE_SNAPSHOT_LIBRARY}",
             gradleArgs,
         )
 

library/src/examplesTest/kotlin/com/gabrielfeo/develocity/api/example/gradle/ResourceInitScripts.kt

@@ -2,4 +2,5 @@ package com.gabrielfeo.develocity.api.example.gradle
 
 object ResourceInitScripts {
     const val FORCE_SNAPSHOT_LIBRARY = "force-snapshot-library.init.gradle.kts"
+    const val REQUIRE_JAVA_11_COMPATIBILITY = "require-java-11-compatibility.init.gradle"
 }

library/src/examplesTest/resources/require-java-11-compatibility.init.gradle

@@ -0,0 +1,11 @@
+afterProject {
+  it.java {
+    sourceCompatibility = JavaVersion.VERSION_11
+    targetCompatibility = JavaVersion.VERSION_11
+  }
+  it.tasks.named('compileKotlin') {
+    compilerOptions {
+      jvmTarget = org.jetbrains.kotlin.gradle.dsl.JvmTarget.JVM_11
+    }
+  }
+}