ci: bump korthout/backport-action from 3 to 4

Bumps [korthout/backport-action](https://github.com/korthout/backport-action) from 3 to 4.
- [Release notes](https://github.com/korthout/backport-action/releases)
- [Commits](https://github.com/korthout/backport-action/compare/v3...v4)

---
updated-dependencies:
- dependency-name: korthout/backport-action
  dependency-version: '4'
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <support@github.com>

.github/workflows/backport.yml

@@ -37,7 +37,7 @@ jobs:
           ref: ${{ github.event.pull_request.head.sha }}
       - name: Create backport PRs
         id: backport
-        uses: korthout/backport-action@v3
+        uses: korthout/backport-action@v4
         with:
           # See https://github.com/korthout/backport-action#inputs
           github_token: ${{ steps.app-token.outputs.token || secrets.GITHUB_TOKEN }}