ci: make backend update PR branch version-specific (#1155)

Co-authored-by: MilesCranmerBot <milescranmerbot@users.noreply.github.com>

Author: MilesCranmerBot

.github/workflows/update_backend.yml

@@ -40,6 +40,7 @@ jobs:
       - name: "Create PR if necessary"
         uses: peter-evans/create-pull-request@v8
         with:
+          branch: backend-update/v${{ steps.get-latest.outputs.version }}
           title: "Automated update to backend: v${{ steps.get-latest.outputs.version }}"
           body: |
             This PR was automatically generated by the GitHub Action `.github/workflows/update-backend.yml`