build: add java 21 to github workflow

Author: jo-elimu

.github/workflows/gradle-build.yml

@@ -11,7 +11,7 @@ jobs:
     strategy:
       matrix:
         os: [ubuntu-latest, macos-latest, windows-latest]
-        java: [11, 17, 21]
+        java: [17, 21]
     runs-on: ${{ matrix.os }}
     steps:
     - uses: actions/checkout@v4