build: Bump actions/checkout from 5 to 6

Bumps [actions/checkout](https://github.com/actions/checkout) from 5 to 6.
- [Release notes](https://github.com/actions/checkout/releases)
- [Changelog](https://github.com/actions/checkout/blob/main/CHANGELOG.md)
- [Commits](https://github.com/actions/checkout/compare/v5...v6)

---
updated-dependencies:
- dependency-name: actions/checkout
  dependency-version: '6'
  dependency-type: direct:production
  update-type: version-update:semver-major
...

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

.github/workflows/openapi.yaml

@@ -6,7 +6,7 @@ jobs:
     runs-on: ubuntu-latest
     container: openapitools/openapi-generator-cli
     steps:
-    - uses: actions/checkout@v5
+    - uses: actions/checkout@v6
     - name: Validate OpenAPI
       env:
         GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}