ci: upgrade actions/checkout to v6 and force Node.js 24 runtime

- Upgraded `actions/checkout` from v4 to v6 to support Node.js 24 natively.
- Added `FORCE_JAVASCRIPT_ACTIONS_TO_NODE24: true` at the workflow level to resolve deprecation warnings for all other actions.
- This ensures the CI environment is future-proof and avoids warnings related to Node.js 20 removal.

Author: XternalSoft

.github/workflows/tests.yml

@@ -1,5 +1,8 @@
 on: [push, pull_request]
 
+env:
+  FORCE_JAVASCRIPT_ACTIONS_TO_NODE24: true
+
 jobs:
 
   php-intl:
@@ -10,7 +13,7 @@ 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@v4
+      - uses: actions/checkout@v6
         with:
           fetch-depth: 1