Passwords: Deprecate second parameter of wp_new_user_notification().

The second parameter `$plaintext_pass` was removed in [33023] and restored as `$notify` in [33620] with a different behavior. If you have a plugin overriding `wp_new_user_notification()` which hasn't been updated you would get a notification with your username and the password "both".
To prevent this the second parameter is now deprecated and reintroduced as the third parameter.

Adds unit tests.

Props kraftbj, adamsilverstein, welcher, ocean90.
Fixes #33654.

(Don't ask for new pluggables kthxbye)
Built from https://develop.svn.wordpress.org/trunk@34116


git-svn-id: http://core.svn.wordpress.org/trunk@34084 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-admin/includes/user.php

@@ -176,7 +176,7 @@ function edit_user( $user_id = 0 ) {
 		$user_id = wp_update_user( $user );
 	} else {
 		$user_id = wp_insert_user( $user );
-		wp_new_user_notification( $user_id, 'both' );
+		wp_new_user_notification( $user_id, null, 'both' );
 	}
 	return $user_id;
 }