Test with Java 21 (#2831)

Author: mpkorstanje

.github/workflows/test-java.yml

@@ -19,7 +19,7 @@ jobs:
     strategy:
       matrix:
         os: [ ubuntu-latest, windows-latest ]
-        version: [ 17, 19 ]
+        version: [ 17, 21 ]
     name: 'Build Java ${{ matrix.version }} - ${{ matrix.os }}'
     runs-on: ${{ matrix.os }}
     steps: