Add post modified fields to WXR export format

[jmdodd]

The WXR export format can be used to merge blogs' content without needing to resort to direct database manipulation. However, its usefulness is limited when doing so because the importer cannot programmatically determine which version of a post is "fresher" as the only date information included with a given post is the post date.

I would like to add the post_modified and post_modified_gmt fields to the generated WXR export file to allow for the possibility of import logic involving updating and adding revisions to, rather than skipping, existing posts/pages.

I have tested these additions with the WordPress importer with no issues in either export or import.

[SergeyBiryukov]

In 49910:

Export: Add post_modified and post_modified_gmt fields to the generated WXR export file.

This allows for more flexibility when determining which version of a post is the latest one, and makes it possible to implement import logic involving updating and adding revisions to existing posts or pages.

Props jmdodd.
Fixes #52180.

[audrasjb]

It would be nice to mention this change in the Miscellaneous Changes dev note.

[audrasjb]

Dev note published: ​https://make.wordpress.org/core/2021/02/23/enhancements-to-the-import-export-feature-in-wordpress-5-7/