chore: bump actions/checkout to v6 for Node.js 24 support

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: jschwendener

.github/workflows/run-tests.yml

@@ -28,7 +28,7 @@ jobs:
                 php-version: ["8.2", "8.3", "8.4"]
 
         steps:
-            - uses: actions/checkout@v4
+            - uses: actions/checkout@v6
 
             - name: Setup PHP
               uses: shivammathur/setup-php@v2