Upgrade actions/checkout to v3

Author: casperbakker

.github/workflows/phpunit.yml

@@ -12,7 +12,7 @@ jobs:
         php-versions: ['7.3', '7.4', '8.0', '8.1', '8.2']
 
     steps:
-    - uses: actions/checkout@v2
+    - uses: actions/checkout@v3
 
     - name: Setup PHP
       uses: shivammathur/setup-php@v2