Update the client API to be consistent with how validation is handled

app/Http/Requests/Api/Client/Servers/Subusers/StoreSubuserRequest.php

@@ -6,10 +6,7 @@ use Pterodactyl\Models\Permission;
 
 class StoreSubuserRequest extends SubuserRequest
 {
-    /**
-     * @return string
-     */
-    public function permission()
+    public function permission(): string
     {
         return Permission::ACTION_USER_CREATE;
     }