ci/backport: update for checkout v7

We need to specify allow-unsafe-pr-checkout now, to be explicit about
checking out PR code on a privileged workflow.

.github/workflows/backport.yml

@@ -38,6 +38,9 @@ jobs:
         with:
           token: ${{ steps.app-token.outputs.token }}
           ref: ${{ github.event.pull_request.head.sha }}
+          # Unsafely checkout the PR HEAD.
+          # backport-action does not execute any code from the checkout.
+          allow-unsafe-pr-checkout: true
 
       - name: Create backport PRs
         id: backport