Fix AbstractFormLoginAuthenticator return types (fixes #47571).

Children of AbstractFormLoginAuthenticator should be allowed to return a `Response` instead of being required to return a `RedirectResponse`.
This change matches the return types in the newer AbstractLoginFormAuthenticator.

Authenticator/AbstractFormLoginAuthenticator.php

@@ -13,6 +13,7 @@
 
 use Symfony\Component\HttpFoundation\RedirectResponse;
 use Symfony\Component\HttpFoundation\Request;
+use Symfony\Component\HttpFoundation\Response;
 use Symfony\Component\Security\Core\Exception\AuthenticationException;
 use Symfony\Component\Security\Core\Security;
 use Symfony\Component\Security\Guard\AbstractGuardAuthenticator;
@@ -34,7 +35,7 @@ abstract protected function getLoginUrl();
     /**
      * Override to change what happens after a bad username/password is submitted.
      *
-     * @return RedirectResponse
+     * @return Response
      */
     public function onAuthenticationFailure(Request $request, AuthenticationException $exception)
     {
@@ -56,7 +57,7 @@ public function supportsRememberMe()
      * Override to control what happens when the user hits a secure page
      * but isn't logged in yet.
      *
-     * @return RedirectResponse
+     * @return Response
      */
     public function start(Request $request, AuthenticationException $authException = null)
     {