workflows: fix checkout in backports action (#2623)

.github/workflows/backport.yml

@@ -9,8 +9,6 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v3
-        with:
-          ref: ${{ github.event.pull_request.head.sha }}
       - name: Create backport PRs
         uses: zeebe-io/backport-action@v0.0.8
         with: