ci: update required backport workflow permissions

.github/workflows/backport.yml

@@ -9,6 +9,9 @@ jobs:
   backport:
     name: Backport
     runs-on: ubuntu-latest
+    permissions:
+      contents: write
+      pull-requests: write
     # Only react to merged PRs for security reasons.
     # See https://docs.github.com/en/actions/using-workflows/events-that-trigger-workflows#pull_request_target.
     if: >