Add java 21 to build matrix

Author: Philippus

Diff for: .github/workflows/ci.yml

@@ -12,7 +12,7 @@ jobs:
     name: Build and Run
     strategy:
       matrix:
-        java-version: [8, 11, 17]
+        java-version: [8, 11, 17, 21]
     runs-on: ubuntu-latest
     steps:
       - name: Checkout current branch (full)