Update Frontend CI to Use JDK 21 (opensearch-project#798)

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>

Author: kaituo

.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: