[D9] Users must verify email when changing user email addresses

Description of the need

When a user changes their email address, Backdrop should send an email with a unique link to the user with the new email address so that the user is required to confirm the change.

Inspired by https://www.drupal.org/project/drupal/issues/85494

This is the patch that would be the closest to what will work here: https://www.drupal.org/project/drupal/issues/85494#comment-450598.

In reading the drupal.org comments, this change should not allow someone to change both the password and the email address at the same time. And there should ideally be a way to keep sending notifications to the original email address until the change is confirmed.