Bump Java to 21 (#1842)

Signed-off-by: Derek Ho <[email protected]>

.github/workflows/opensearch-observability-test-and-build-workflow.yml

@@ -14,7 +14,7 @@ jobs:
       # Run all jobs
       fail-fast: false
       matrix:
-        java: [11, 17, 21]
+        java: [21]
     runs-on: ubuntu-latest
     container:
       # using the same image which is used by opensearch-build team to build the OpenSearch Distribution
@@ -23,6 +23,9 @@ jobs:
       # need to switch to root so that github actions can install runner binary on container without permission issues.
       options: --user root
 
+    env:
+      ACTIONS_ALLOW_USE_UNSECURE_NODE_VERSION: true
+
     steps:
       - uses: actions/checkout@v1
 
@@ -65,7 +68,7 @@ jobs:
       # Run all jobs
       fail-fast: false
       matrix:
-        java: [11, 17, 21]
+        java: [21]
         os: [windows-latest, macos-latest]
     runs-on: ${{ matrix.os }}