Include value of EMPTY_TRASH_DAYS constant

[desrosj]

The EMPTY_TRASH_DAYS constant controls how long posts and comments remain in the trash before wp_scheduled_delete() deletes them permanently.

The default value is 30, but setting the constant to a falsey value will skip the trash entirely any time delete post/comment is clicked. This value could be useful for debugging certain behaviors in a WordPress site.

This PR introduces visibility for the EMPTY_TRASH_DAYS constant in the Site Health “WordPress Constants” section.
It helps site administrators quickly identify how long trashed content is retained before permanent deletion — or whether the trash feature is disabled.

Trac ticket: https://core.trac.wordpress.org/ticket/64189

Screenshot

[SergeyBiryukov]

In 61359:

Site Health: Include value of EMPTY_TRASH_DAYS constant in debug data.

Follow-up to [44986], [45078], [58855].

Props deepakprajapati, desrosj, dhruvang21, SergeyBiryukov.
Fixes #64189.