test: Do not use PHP 8 keywords yet.

It's much too early ! :)

src/Bundle/ChillWopiBundle/src/Service/Controller/ResponderInterface.php

@@ -35,11 +35,12 @@ interface ResponderInterface
     /**
      * Returns a JsonResponse that uses the serializer component if enabled, or json_encode.
      *
+     * @param mixed $data
      * @param array<string, list<string>|string> $headers
      * @param array<string, mixed> $context
      */
     public function json(
-        mixed $data,
+        $data,
         int $status = 200,
         array $headers = [],
         array $context = []