ci: Update ci.yaml to introduce Java 21 unit testing (#1907)

Author: ddixit14

synthtool/gcp/templates/java_library/.github/workflows/ci.yaml

@@ -25,7 +25,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        java: [11, 17]
+        java: [11, 17, 21]
     steps:
     - uses: actions/checkout@v3
     - uses: actions/setup-java@v3