Update AbstractController.php

Removing `string` type-hint of $message at addFlash()

Closes #28991 and #34645

Reasons:

* `addFlash()` is just a convenience shortcut for `FlashBagInterface::add()` which doesn't have the type hint: https://github.com/symfony/symfony/blob/master/src/Symfony/Component/HttpFoundation/Session/Flash/FlashBagInterface.php#L28 . So removing it here improves consistency.

* #28991 (comment) is a valid use case for having an object as `$message`.

* Twig doesn't have any rendering helpers for the `message`, see https://symfony.com/doc/current/controller.html#flash-messages . And since users have to take care of displaying the `message` themselves, there's no reason to force a string upon them.

src/Symfony/Bundle/FrameworkBundle/Controller/ControllerTrait.php

@@ -187,7 +187,7 @@ protected function file($file, $fileName = null, $disposition = ResponseHeaderBa
      * Adds a flash message to the current session for type.
      *
      * @param string $type    The type
-     * @param string $message The message
+     * @param mixed  $message The message
      *
      * @throws \LogicException
      *