Add Java 21 CI

Author: fwcd

.github/workflows/build.yml

@@ -13,7 +13,7 @@ jobs:
     strategy:
       matrix:
         os: [ubuntu-latest, macos-latest, windows-latest]
-        java: [ '11', '17' ]
+        java: ['11', '17', '21']
     steps:
       - uses: actions/checkout@v4
       - name: Setup JDK