[TASK] Adjust GH workflow for write permission (#126)

Co-authored-by: Garvin Hicking <garvin@hick.ing>

Author: github-actions[bot]

.github/workflows/backport.yml

@@ -9,6 +9,9 @@ jobs:
   backport:
     runs-on: ubuntu-latest
     name: Backport
+    permissions:
+      contents: write        # needed to push the backport branch
+      pull-requests: write   # needed to open the PR
     steps:
       - name: Backport
         uses: m-kuhn/backport@v1.2.6