Fix PasswordRehash default config to use authenticators instead of deprecated identifiers

Fixes #1170

The default configuration was using the deprecated 'identifiers' key which causes
a warning because the Password identifier is nested inside the Form authenticator,
not directly in the authentication service.

Changed from:
  'PasswordRehash' => ['identifiers' => ['Password']]

To:
  'PasswordRehash' => ['authenticators' => ['Form' => 'Password']]

Author: steinkel

config/users.php

@@ -284,7 +284,9 @@
         ],
         'RbacPolicy' => [],
         'PasswordRehash' => [
-            'identifiers' => ['Password'],
+            'authenticators' => [
+                'Form' => 'Password',
+            ],
         ],
     ],
     'OAuth' => [