Skip checkstyle on Java < 21 in CI

Checkstyle 13.0.0 requires Java 21+. Skip checkstyle when running
tests on Java 17 since code style is independent of Java version.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: horgh

.github/workflows/test.yml

@@ -19,7 +19,15 @@ jobs:
         with:
           distribution: ${{ matrix.distribution }}
           java-version: ${{ matrix.version }}
-      - run: mvn test -B
+      # Checkstyle 13+ requires Java 21+. Skip on older JDKs.
+      - name: Run tests
+        run: |
+          if [ "${{ matrix.version }}" -lt 21 ]; then
+            mvn test -B -Dcheckstyle.skip=true
+          else
+            mvn test -B
+          fi
+        shell: bash
       # This is after the test run to work around
       # https://issues.apache.org/jira/projects/MJAVADOC/issues/MJAVADOC-736
       - run: mvn javadoc:javadoc