gha: bump actions/checkout

Fixes
> `Node.js 16 actions are deprecated. Please update the following actions to use Node.js 20: actions/checkout@v2, actions/cache@v2. For more information see: https://github.blog/changelog/2023-09-22-github-actions-transitioning-from-node-16-to-node-20/.`

.github/workflows/php-cs-fixer.yml

@@ -14,7 +14,7 @@ jobs:
 
     steps:
       - name: Checkout code
-        uses: actions/checkout@v2
+        uses: actions/checkout@v4
 
       - name: Setup PHP
         uses: shivammathur/setup-php@v2

.github/workflows/phpstan.yml

@@ -27,7 +27,7 @@ jobs:
       COMPOSER_NO_INTERACTION: 1
 
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v4
 
       - uses: shivammathur/setup-php@v2
         with:

.github/workflows/phpunit.yml

@@ -75,7 +75,7 @@ jobs:
 
     steps:
       - name: Checkout code
-        uses: actions/checkout@v1
+        uses: actions/checkout@v4
 
       - name: Setup PHP
         uses: shivammathur/setup-php@v2