ci: Update ci.yaml to introduce Java 21 unit testing (#1907) (#1837)

Source-Link: googleapis/synthtool@c06d605
Post-Processor: gcr.io/cloud-devrel-public-resources/owlbot-java:latest@sha256:6f431774e11cc46619cf093fd1481193c4024031073697fa18f0099b943aab88

Co-authored-by: Owl Bot <gcf-owl-bot[bot]@users.noreply.github.com>

Author: gcf-owl-bot[bot]

.github/.OwlBot.lock.yaml

@@ -13,5 +13,5 @@
 # limitations under the License.
 docker:
   image: gcr.io/cloud-devrel-public-resources/owlbot-java:latest
-  digest: sha256:bc2bf47c7370f1b1a8a46b0c104ce7e43644ac58902c9de265fe1f253fcc2506
-# created: 2023-10-31T13:44:21.909247346Z
+  digest: sha256:6f431774e11cc46619cf093fd1481193c4024031073697fa18f0099b943aab88
+# created: 2023-12-01T19:50:20.444857406Z

.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