Bump actions/checkout from 4 to 5 (OpenAPITools#21889)

Bumps [actions/checkout](https://github.com/actions/checkout) from 4 to 5.
- [Release notes](https://github.com/actions/checkout/releases)
- [Changelog](https://github.com/actions/checkout/blob/main/CHANGELOG.md)
- [Commits](actions/checkout@v4...v5)

---
updated-dependencies:
- dependency-name: actions/checkout
  dependency-version: '5'
  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/samples-dart-build-test.yaml

@@ -34,7 +34,7 @@ jobs:
           - samples/openapi3/client/petstore/dart-dio/oneof_polymorphism_and_inheritance/
           - samples/openapi3/client/petstore/dart-dio/binary_response/
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v5
 
       - uses: dart-lang/setup-dart@v1
         with: