php.yml: Update to actions/cache@v4

Fixes #66

Author: reedy

.github/workflows/php.yml

@@ -34,7 +34,7 @@ jobs:
       run: composer validate
 
     - name: Cache Composer cache
-      uses: actions/cache@v2
+      uses: actions/cache@v4
       with:
         path: ~/.composer/cache
         key: composer-cache-php${{ matrix.php }}