chore: add ci testruns for PHP > 8.0 (thephpleague#263)

fix deprecation of github action versions

Co-authored-by: Christopher Georg <christopher.georg@sr-travel.de>

Author: Chris53897

.github/workflows/phpunit.yml

@@ -30,7 +30,7 @@ jobs:
 
     steps:
       - name: "Checkout"
-        uses: "actions/checkout@v2"
+        uses: "actions/checkout@v4"
 
       - name: "Install PHP"
         uses: "shivammathur/setup-php@v2"