Use "io.spring.nullability" Gradle plugin

Closes gh-1232

Author: bclozel

build.gradle

@@ -1,3 +1,7 @@
+plugins {
+	id "io.spring.nullability" version "0.0.1" apply false
+}
+
 description = "Spring for GraphQL"
 
 ext {
@@ -10,7 +14,7 @@ ext {
 subprojects {
 	apply plugin: 'org.springframework.graphql.conventions'
 	apply plugin: 'org.springframework.graphql.architecture'
-	apply plugin: 'org.springframework.graphql.nullability'
+	apply plugin: "io.spring.nullability"
 	group = 'org.springframework.graphql'
 
 	ext.javadocLinks = [

buildSrc/build.gradle

@@ -41,10 +41,6 @@ gradlePlugin {
 			id = "org.springframework.graphql.architecture"
 			implementationClass = "org.springframework.graphql.build.architecture.ArchitecturePlugin"
 		}
-		nullability {
-			id = "org.springframework.graphql.nullability"
-			implementationClass = "org.springframework.graphql.build.nullability.NullabilityPlugin"
-		}
 	}
 }