chore: add jdk 21 job for showcase tests (#2694)

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Fixes #<issue_number_goes_here> ☕️

Co-authored-by: Lawrence Qiu <lawrenceqiu@google.com>

.github/workflows/ci.yaml

@@ -211,7 +211,7 @@ jobs:
     runs-on: ubuntu-22.04
     strategy:
       matrix:
-        java: [ 11, 17 ]
+        java: [ 11, 17, 21 ]
     steps:
       - uses: actions/checkout@v4
       - uses: actions/setup-java@v4