add legacy tag

Author: jrushlow

src/Maker/MakeRegistrationForm.php

@@ -274,6 +274,10 @@ public function generate(InputInterface $input, ConsoleStyle $io, Generator $gen
             'Controller\\'
         );
 
+        /*
+         * @legacy Conditional can be removed when MakerBundle no longer
+         *         supports Symfony < 6.0
+         */
         $passwordHasher = UserPasswordEncoderInterface::class;
 
         if (interface_exists(UserPasswordHasherInterface::class)) {