Attempt to delete backport branches only for OpenSearch repo (#16240)

Signed-off-by: Sayali Gaikawad <[email protected]>

Author: gaiksaya

.github/workflows/delete_backport_branch.yml

@@ -9,7 +9,7 @@ jobs:
     runs-on: ubuntu-latest
     permissions:
       contents: write
-    if: startsWith(github.event.pull_request.head.ref,'backport/')
+    if: github.repository == 'opensearch-project/OpenSearch' && startsWith(github.event.pull_request.head.ref,'backport/')
     steps:
     - name: Delete merged branch
       uses: actions/github-script@v7