chore(deps): update actions/checkout action to v6

Author: renovate[bot]

.github/workflows/ci.yml

@@ -28,7 +28,7 @@ jobs:
             phpunit: '13'
 
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
 
       - name: Setup PHP
         uses: shivammathur/setup-php@v2
@@ -48,7 +48,7 @@ jobs:
     name: PHPStan
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
 
       - name: Setup PHP
         uses: shivammathur/setup-php@v2
@@ -66,7 +66,7 @@ jobs:
     name: PHP-CS-Fixer
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
 
       - name: Setup PHP
         uses: shivammathur/setup-php@v2