last_updated vs last_modified_at for posts

[sim642]

Have you checked that your feature request isn't already filed?

• I read through FAQ and searched through the past issues, none of which addressed my feature request.
• Yes, I have checked that this feature request isn't already filed.

Description & Motivation

#2341 added the last_updated field to posts. However, jekyll-sitemap, which is included here, uses last_modified_at instead. This requires duplicating the date in two fields.

Pitch

Transition to last_modified_at field instead:

1. Use last_modified_at to output what last_updated is used for right now.
2. Direct new users to use last_modified_at for that purpose.

For backwards-compatibility last_updated should be used as fallback.

Alternatives

No response

Additional context

No response

[george-gca]

Could you send a PR for this?