[SecurityBundle] Add directive deprecated for custom_authenticators key when there is more than one registered provider

Author: mdoutreluingne

security/custom_authenticator.rst

@@ -130,6 +130,12 @@ The authenticator can be enabled using the ``custom_authenticators`` setting:
             ;
         };
 
+.. deprecated:: 5.4
+
+    Having multiple user providers and a ``custom_authenticator`` and not having specified the user provider 
+    to use has been deprecated in Symfony 5.4. If you have registered multiple user providers, you must set the 
+    ``provider`` key to one of the configured providers, even if your custom authenticators don't use it.
+
 .. versionadded:: 5.2
 
     Starting with Symfony 5.2, the custom authenticator is automatically