minor #18451 Fix return type on custom authenticators (FabienPapet)

This PR was submitted for the 6.4 branch but it was merged into the 6.2 branch instead.

Discussion
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Fix return type on custom authenticators

The return type of this part of the documentation is giving a class that has been deprecated in Symfony 5.3

Commits
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7da82f8 Update custom_authenticator.rst

Author: xabbuh

security/custom_authenticator.rst

@@ -239,7 +239,7 @@ using :ref:`the user provider <security-user-providers>`::
                 // ...
 
                 return new Passport(
-                    new UserBadge($email, function (string $userIdentifier): ?User {
+                    new UserBadge($email, function (string $userIdentifier): ?UserInterface {
                         return $this->userRepository->findOneBy(['email' => $userIdentifier]);
                     }),
                     $credentials