Update GitHub Actions to resolve CI build failures

- Update actions/checkout from v2 to v4
- Update actions/cache from v2 to v4 (fixes deprecation warning)
- Keep shivammathur/setup-php@v2 (already current)

This resolves the CI failure: "This request has been automatically failed because it uses a deprecated version of actions/cache: v2"

Co-authored-by: lisachenko <[email protected]>

Author: Copilot

.github/workflows/phpunit.yml

@@ -23,7 +23,7 @@ jobs:
 
     steps:
       - name: "Checkout"
-        uses: "actions/checkout@v2"
+        uses: "actions/checkout@v4"
 
       - name: "Install PHP"
         uses: "shivammathur/setup-php@v2"
@@ -34,7 +34,7 @@ jobs:
           tools: composer:v2, cs2pr
 
       - name: "Cache dependencies"
-        uses: "actions/cache@v2"
+        uses: "actions/cache@v4"
         with:
           path: |
             ~/.composer/cache