Update actions/checkout action to v6 (#24)

Co-authored-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>

Author: renovate[bot]

.github/workflows/ci.yml

@@ -15,7 +15,7 @@ jobs:
     runs-on: ubuntu-latest
     name: PHP ${{ matrix.php }} - ${{ matrix.prefer-lowest == 1 && 'Lowest' || 'Latest' }} Dependencies
     steps:
-    - uses: actions/checkout@v5
+    - uses: actions/checkout@v6
     - uses: RevoTale/php-composer-testing-action@v1
       with:
         php-version: ${{ matrix.php }}
@@ -25,7 +25,7 @@ jobs:
   Rector:
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@v5
+    - uses: actions/checkout@v6
     - uses: RevoTale/php-composer-testing-action@v1
       with:
         php-version: 8.4
@@ -35,7 +35,7 @@ jobs:
   PHPStan:
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@v5
+    - uses: actions/checkout@v6
     - uses: RevoTale/php-composer-testing-action@v1
       with:
         php-version: 8.4