[DX][D9] Add the ability to deprecate the name (and value?) of entries in settings.php

This is basically the equivalent of https://www.drupal.org/project/drupal/issues/3163226 (as well as a sibling issue to #5741) which was introduced in D 9.1.x (Change record: Core settings keys can be deprecated)

If the settings.php file for a site (or any file it includes) contains the legacy setting name, the deprecation warning will be generated. If code calls Settings::get('new_setting') but 'new_setting' is not yet defined, the value of 'old_setting' will be returned.

If any code calls Settings::get() with the legacy name, the deprecation warning will also be generated. If the replacement setting is already defined, the value returned by Settings::get('old_setting') will be the value of new_setting.

This would help us with issues similar to #4451 as well, and possibly allow us to deprecate things in the Backdrop 1.x release cycle instead of having to wait for 2.x.