Update Java version to 21 in GitHub Actions workflow

Author: wikumChamith

.github/workflows/maven.yml

@@ -13,7 +13,7 @@ jobs:
     strategy:
       matrix:
         platform: [ ubuntu-latest ]
-        java-version: [ 8 ]
+        java-version: [ 21 ]
 
     runs-on: ${{ matrix.platform }}
     env: