Auto-update `.htaccess`, `robots.txt`, or `settings.php` (when we can)

Description of the need

When .htaccess, robots.txt, or settings.php have been updated in a release, we should update them when safe/possible

Proposed solution

We could store a hash of the previous version of the file in state. When doing an update, check the hash in state to see if the contents of the file has been changed. If not, we can overwrite it (assuming Backdrop has permissions to write to files outside of core) and update the hash with the hash of the new file.

Alternatives that have been considered

• https://github.com/backdrop/backdrop-issues/issues/5911 (but I think we should do both)

Draft of feature description for Press Release (1 paragraph at most)

Backdrop core can now automatically update files outside the core directory -- assuming they have not been previously modified and that Backdrop has permission to write to those files.