Show notice in the admin UI when .htaccess, robots.txt, or settings.php have been updated in a release, and either link to instructions, or auto-update when safe/possible

Description of the need

The documentation for updates is based around a manual update on the server. However we have an updater in the UI, which many people will use to update Backdrop.

The upcoming release, 1.24.0 includes an update to the .htaccess file in the root directory (likely to have changes) and the .htaccess file in files/ and the release notes for the preview state: 'The .htaccess file has been modified to include settings for PHP 8. (See #5906)'

Site owner who just use the updater might not see these release notes and may not know. @klonos has suggested on Zulip that the updater will only update the core folder so at least customisations won't be broken, but how should they get the updates?

Proposed solution

Site owner should be guided through updating the files without losing their customisations. Not sure how best to do this.