User title callback should use getDisplayName instead of getUsername

As a developer, I expect hook_user_format_name_alter() to alter the name displayed in the page titles for users according to the docs for AccountInterface. However, that does not work as expected because the _title_callback in UserRouteProvider calls userTitle() on the UserController, which uses the deprecated getUsername() method to retrieve the name to display.

Changing the userTitle() method to call getDisplayName() results in no change to the default behavior of the title callback.