Add PHP 8.4 to test matrix

Author: gapple

.github/workflows/php.yml

@@ -17,10 +17,10 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        php-versions: ['8.1', '8.2', '8.3']
+        php-versions: ['8.1', '8.2', '8.3', '8.4']
         experimental: [false]
     env:
-      code-coverage-version: '8.3' # Most recent stable PHP version.
+      code-coverage-version: '8.4' # Most recent stable PHP version.
 
     name: PHP ${{ matrix.php-versions }}