Added Java 17 and 21 for CI builds

Author: fschleich

.github/workflows/nebula-ci.yml

@@ -12,8 +12,8 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        # test against JDK 11 
-        java: [ 11 ]
+        # test against JDK 11, 17 and 21 
+        java: [ 11, 17, 21 ]
     name: CI with Java ${{ matrix.java }}
     steps:
       - uses: actions/checkout@v1