chore: Update ci.yaml to setup Java 21 unit testing. (googleapis#1910)

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

.github/workflows/ci.yaml

@@ -87,3 +87,27 @@ jobs:
     - run: .kokoro/build.sh
       env:
         JOB_TYPE: clirr
+  # compilation failure for sub-modules using source and target options 7 (this  setting cannot be upgraded to Java 21 because some modules support max of Java 8) 
+  # Hence compile in Java 8 and test in Java 21.    
+  units-java21:
+    # Building using Java 8 and run the tests with Java 21 runtime
+    name: "units (21)"
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v3
+      - uses: actions/setup-java@v3
+        with:
+          java-version: 21
+          distribution: temurin
+      - name: "Set jvm system property environment variable for surefire plugin (unit tests)"
+        # Maven surefire plugin (unit tests) allows us to specify JVM to run the tests.
+        # https://maven.apache.org/surefire/maven-surefire-plugin/test-mojo.html#jvm
+        run: echo "SUREFIRE_JVM_OPT=-Djvm=${JAVA_HOME}/bin/java" >> $GITHUB_ENV
+        shell: bash
+      - uses: actions/setup-java@v3
+        with:
+          java-version: 8
+          distribution: temurin
+      - run: .kokoro/build.sh
+        env:
+          JOB_TYPE: test