chore: fix php_versions

Author: imorland

.github/workflows/backend.yml

@@ -8,6 +8,6 @@ jobs:
     with:
       enable_backend_testing: true
       enable_phpstan: true
-      php_versions: '["7.4", "8.0, "8.1", "8.2", "8.3", "8.4"]'
+      php_versions: '["7.4", "8.0", "8.1", "8.2", "8.3", "8.4"]'
 
       backend_directory: .