Enhance changelog workflow to check for missing labels (opensearch-project#13167)

Signed-off-by: Kunal Kotwani <kkotwani@amazon.com>

Author: kotwanikunal

.github/workflows/changelog_verifier.yml

@@ -26,6 +26,19 @@ jobs:
           changeLogPath: 'CHANGELOG.md'
         continue-on-error: true
       - run: |
+          # The check was possibly skipped leading to success for both the jobs
+          if [[ ${{ steps.verify-changelog-3x.outcome }} == 'success' && ${{ steps.verify-changelog.outcome }} == 'success' ]]; then
+            exit 0
+          fi
+          
           if [[ ${{ steps.verify-changelog-3x.outcome }} == 'failure' && ${{ steps.verify-changelog.outcome }} == 'failure' ]]; then
+            echo "error: Please ensure a changelog entry exists in CHANGELOG.md or CHANGELOG-3.0.md"
+            exit 1
+          fi
+          
+          # Concatenates the labels and checks if the string contains "backport"
+          has_backport_label=${{ contains(join(github.event.pull_request.labels.*.name, ', '), 'backport')}}
+          if [[ ${{ steps.verify-changelog.outcome }} == 'success' && $has_backport_label == false ]]; then
+            echo "error: Please make sure that the PR has a backport label associated with it when making an entry to the CHANGELOG.md file"
             exit 1
           fi