update GitHub workflow to use the latest checkout

Author: pdscopes

.github/workflows/phpunit.yml

@@ -12,7 +12,7 @@ jobs:
     runs-on: ubuntu-latest
 
     steps:
-    - uses: actions/checkout@v2
+    - uses: actions/checkout@v3
 
     - name: Validate composer.json and composer.lock
       run: composer validate --strict