ci: bump actions/stale from 6 to 9 (#67)

Bumps [actions/stale](https://github.com/actions/stale) from 6 to 9.
- [Release notes](https://github.com/actions/stale/releases)
- [Changelog](https://github.com/actions/stale/blob/main/CHANGELOG.md)
- [Commits](actions/stale@v6...v9)

---
updated-dependencies:
- dependency-name: actions/stale
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

.github/workflows/mark-stale.yml

@@ -14,7 +14,7 @@ jobs:
 
     steps:
     - name: Mark Stale
-      uses: actions/stale@v6
+      uses: actions/stale@v9
       with:
         repo-token: ${{ secrets.GITHUB_TOKEN }}
         exempt-all-milestones: true