Bump actions/checkout from 3 to 4

Bumps [actions/checkout](https://github.com/actions/checkout) from 3 to 4.
- [Release notes](https://github.com/actions/checkout/releases)
- [Changelog](https://github.com/actions/checkout/blob/main/CHANGELOG.md)
- [Commits](actions/checkout@v3...v4)

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

Signed-off-by: dependabot[bot] <[email protected]>

.github/workflows/gateway-conformance.yml

@@ -58,7 +58,7 @@ jobs:
 
       # 4. Build rainbow
       - name: Checkout rainbow
-        uses: actions/checkout@v3
+        uses: actions/checkout@v4
         with:
           path: rainbow
       - name: Build rainbow