Add more specific @psalm-assert condition

Since psalm issue (vimeo/psalm#2409) resolved, we can use union types in assertions

Author: andrew-demb

src/Symfony/SymfonyMessageSerializer.php

@@ -161,7 +161,7 @@ public function normalize(object $message): array
     }
 
     /**
-     * @psalm-assert array{message:array<string, mixed>, namespace:class-string} $data
+     * @psalm-assert array{message:array<string, string|int|float|null>, namespace:class-string} $data
      *
      * @throws \UnexpectedValueException
      */