ci: upgrade actions/checkout to v4 for Node.js 24 compatibility

- Updated `actions/checkout` from v2 to v4 to resolve Node.js 20 deprecation warnings.
- Ensures the CI workflow remains functional on newer GitHub Actions runners.

Author: XternalSoft

.github/workflows/tests.yml

@@ -10,7 +10,7 @@ jobs:
         php-version: ['7.2', '7.3', '7.4', '8.0', '8.1', '8.2', '8.3', '8.4', '8.5']
 
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v4
         with:
           fetch-depth: 1