Upgrade actions/checkout

Author: stefanzweifel

.github/workflows/run-tests.yml

@@ -15,7 +15,7 @@ jobs:
 
     steps:
       - name: Checkout code
-        uses: actions/checkout@v2
+        uses: actions/checkout@v3
 
       - name: Setup PHP
         uses: shivammathur/setup-php@v2