chore: upgrade actions/checkout to v6 (Node.js 22) (#957)

Resolves Node.js 20 deprecation warning by upgrading from
actions/checkout@v4 to actions/checkout@v6 which runs on Node.js 22.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: jordyvanderhaegen

.github/workflows/run-tests.yml

@@ -34,7 +34,7 @@ jobs:
 
     steps:
       - name: Checkout code
-        uses: actions/checkout@v4
+        uses: actions/checkout@v6
 
       - name: Setup PHP
         uses: shivammathur/setup-php@v2