Bump actions/checkout from 2 to 3 (prometheus-community#9)

Bumps [actions/checkout](https://github.com/actions/checkout) from 2 to 3.
- [Release notes](https://github.com/actions/checkout/releases)
- [Changelog](https://github.com/actions/checkout/blob/main/CHANGELOG.md)
- [Commits](actions/checkout@v2...v3)

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

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

.github/workflows/sync-readme.yaml

@@ -15,7 +15,7 @@ jobs:
       - uses: actions/checkout@v3
       - run: |
           cp -f README.md ${{ runner.temp }}/README.md
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v3
         with:
           ref: gh-pages
       - run: |