Test with JDK 21 on GitHub Actions

Author: tbroyer

.github/workflows/maven.yml

@@ -78,8 +78,8 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        java: [ '11', '17', '20' ]
-    continue-on-error: ${{ matrix.java == '20' }}
+        java: [ '11', '17', '21' ]
+    continue-on-error: ${{ matrix.java == '21' }}
     steps:
       - uses: actions/checkout@v4