minor #17641 [Security] Prevent unneeded implementation of PasswordHasherAwareInterface when migrating passwords (MatTheCat)

This PR was squashed before being merged into the 6.2 branch.

Discussion
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[Security] Prevent unneeded implementation of `PasswordHasherAwareInterface` when migrating passwords

Trying to avoid issues like symfony/symfony#48348 or symfony/symfony#48825

Commits
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1d09493 [Security] Prevent unneeded implementation of `PasswordHasherAwareInterface` when migrating passwords

Author: javiereguiluz

security/passwords.rst

@@ -123,7 +123,7 @@ Further in this article, you can find a
     .. configuration-block::
 
         .. code-block:: yaml
-        
+
             # config/packages/test/security.yaml
             security:
                 # ...
@@ -533,8 +533,10 @@ migration by returning ``true`` in the ``needsRehash()`` method::
         }
     }
 
-Named Password Hashers
-----------------------
+.. _named-password-hashers:
+
+Dynamic Password Hashers
+------------------------
 
 Usually, the same password hasher is used for all users by configuring it
 to apply to all instances of a specific class. Another option is to use a
@@ -635,6 +637,12 @@ the name of the hasher to use::
         }
     }
 
+.. caution::
+
+    When :ref:`migrating passwords <security-password-migration>`, you don't need to implement ``PasswordHasherAwareInterface``
+    to return the legacy hasher name:
+    Symfony will detect it from your ``migrate_from`` configuration.
+
 If you created your own password hasher implementing the
 :class:`Symfony\\Component\\PasswordHasher\\PasswordHasherInterface`,
 you must register a service for it in order to use it as a named hasher: