Ensure forward compatibility by executing the tests with Java 17 & 21

Note that the goal is not to require Java 21, just to make sure that
WindowBuilder that it can be executed with the current LTS.

.github/workflows/maven.yaml

@@ -13,15 +13,15 @@ jobs:
       fail-fast: false
       matrix:
         os: [ubuntu-latest, windows-latest]
-        java: [ 17 ]
+        java: [ 17, 21 ]
     runs-on: ${{ matrix.os }}   
     name: OS ${{ matrix.os }} Java ${{ matrix.java }} compile
     timeout-minutes: 90
     steps:
     - uses: actions/checkout@v3
       with:
        fetch-depth: 0
-    - name: Set up JDK 11
+    - name: Set up JDK 17/21
       uses: actions/setup-java@v3
       with:
         distribution: 'temurin' # See 'Supported distributions' for available options