Bump actions/checkout from 4 to 6

Bumps [actions/checkout](https://github.com/actions/checkout) from 4 to 6.
- [Release notes](https://github.com/actions/checkout/releases)
- [Changelog](https://github.com/actions/checkout/blob/main/CHANGELOG.md)
- [Commits](actions/checkout@v4...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>

Author: dependabot[bot]

.github/workflows/ci.yaml

@@ -21,5 +21,5 @@ jobs:
   yamllint:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
       - run: yamllint -f github .

.github/workflows/pytest.yaml

@@ -36,7 +36,7 @@ jobs:
     outputs:
       strategy: ${{ steps.load.outputs.strategy }}
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
         with:
           repository: ${{ inputs.setup-repository }}
       - id: load
@@ -50,13 +50,13 @@ jobs:
     runs-on: ${{ matrix.os }}
 
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
         with:
           repository: ${{ inputs.repository }}
       - uses: actions/setup-python@v5
         with:
           python-version: ${{ matrix.python }}
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
         with:
           repository: ${{ inputs.setup-repository }}
           path: ./.github-ci-action-repo