ci: remove Java 11, add Java 21

Author: netmikey

.github/workflows/build.yaml

@@ -7,7 +7,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        java: [ '11', '17' ]
+        java: [ '17', '21' ]
     name: Java ${{ matrix.java }}
     steps:
       - uses: actions/checkout@v3