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

@@ -10,7 +10,7 @@ jobs:
     env:
       PHP_CS_FIXER_IGNORE_ENV: true
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
       - uses: shivammathur/setup-php@v2
         with:
           php-version: '8.4'
@@ -23,7 +23,7 @@ jobs:
     runs-on: ubuntu-latest
     name: phpstan
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
       - uses: shivammathur/setup-php@v2
         with:
           php-version: '8.4'
@@ -41,7 +41,7 @@ jobs:
         stability: [prefer-lowest, prefer-stable]
     name: PHPUnit on PHP ${{ matrix.php }}@${{ matrix.stability }}
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
       - uses: shivammathur/setup-php@v2
         with:
           php-version: ${{ matrix.php }}
@@ -57,7 +57,7 @@ jobs:
   yamllint:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
       - uses: ibiqlik/action-yamllint@v3
         with:
           format: github