Test jsoar against java 21 as well

Author: marinier

.github/workflows/maven.yml

@@ -14,7 +14,7 @@ jobs:
     strategy:
       matrix:
         os: ['ubuntu-latest', 'windows-latest', 'macos-latest']
-        java: [ 11, 17 ]
+        java: [ 11, 17, 21 ]
     runs-on: ${{ matrix.os }}
     name: Java ${{ matrix.java }} on ${{ matrix.os }}
     steps: