Update Frontend CI to Use JDK 21

This PR updates the frontend CI configuration to set up JDK 21, aligning with the baseline JDK 21 already set on the backend (opensearch-project/anomaly-detection#1228). Without this update, remote tests in CI would fail with the error: "error: release version 21 not supported."

Testing:
* Verified that remote tests in CI pass successfully with JDK 21.

Signed-off-by: Kaituo Li <kaituo@amazon.com>

.github/workflows/remote-integ-tests-workflow.yml

@@ -17,7 +17,7 @@ jobs:
       fail-fast: false
       matrix:
         os: [ ubuntu-latest ]
-        jdk: [ 11 ]
+        jdk: [ 21 ]
     runs-on: ${{ matrix.os }}
 
     steps: