Adding auto delete workflow for backport branches (opensearch-project#176)

Signed-off-by: Vacha Shah <vachshah@amazon.com>

Author: VachaShah

.github/workflows/delete_backport_branch.yml

@@ -0,0 +1,15 @@
+name: Delete merged branch of the backport PRs
+on: 
+  pull_request:
+    types:
+      - closed
+  
+jobs:
+  delete-branch:
+    runs-on: ubuntu-latest
+    if: startsWith(github.event.pull_request.head.ref,'backport/')
+    steps:
+      - name: Delete merged branch
+        uses: SvanBoxel/delete-merged-branch@main
+        env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}