Include system JDK

Author: tkindy

.github/workflows/ci.yml

@@ -16,7 +16,10 @@ jobs:
       - uses: actions/setup-java@v3
         with:
           distribution: "temurin"
-          java-version: ${{ matrix.java }}
+          # Always include 21 as the system JDK
+          java-version: |
+            ${{ matrix.java }}
+            21
           cache: "maven"
       - name: "Build"
         run: mvn --batch-mode -no-transfer-progress -V verify