chore: Update ci.yaml to setup Java 21 CI tests (#2299)

* chore: Update ci.yaml to setup Java 21 unit testing

.bazeliskrc

@@ -1,2 +1,3 @@
 # See https://github.com/bazelbuild/bazelisk
-USE_BAZEL_VERSION=6.0.0
+# As per b/302171264#comment5, 6.4.0 supports Java 21. 
+USE_BAZEL_VERSION=6.4.0

.github/workflows/ci.yaml

@@ -9,7 +9,7 @@ jobs:
     runs-on: ubuntu-22.04
     strategy:
       matrix:
-        java: [ 11, 17 ]
+        java: [ 11, 17]
     steps:
       - uses: actions/checkout@v3
       - uses: actions/setup-java@v3
@@ -48,7 +48,29 @@ jobs:
           pushd /tmp/java-compute/google-cloud-compute-small-v1-java
           ./gradlew clean build publishToMavenLocal sourcesJar allJars
           popd
-
+  build-java-21:
+    name: "build(21) except self-service clients"
+    # Support for Java 21 is available for all use cases except self-service clients.
+    runs-on: ubuntu-22.04
+    steps:
+      - uses: actions/checkout@v3
+      - uses: actions/setup-java@v3
+        with:
+          java-version: 21
+          distribution: temurin
+          cache: maven
+      - run: java -version
+      - name: Unit Tests
+        run: |
+          mvn test --batch-mode --no-transfer-progress -Dcheckstyle.skip \
+              -Dfmt.skip -DenableTestCoverage
+      - run: bazelisk version
+      - name: Install Maven modules
+        run: |
+          mvn install -B -ntp -DskipTests -Dclirr.skip -Dcheckstyle.skip
+      - name: Integration Tests
+        run: |
+          bazelisk --batch test //test/integration/...
   build-java8-except-gapic-generator-java:
     name: "build(8) except for gapic-generator-java"
     runs-on: ubuntu-22.04