Any WordPress user who can `edit_posts` of a post type with `show_in_rest=true` can query for authors. This maps to current WordPress behavior where a WordPress user who can view the Manage Posts view for a post type can see any WordPress user assigned to a post (whether published or draft).
This implementation, over restricting `who=authors` to users with `list_users`, gives us future flexibility in displaying lists of posts. It still respects more restrictive permissions for `context=edit`.
Props danielbachhuber.
Fixes#42202.
git-svn-id: https://develop.svn.wordpress.org/trunk@43001 602fd350-edb4-49c9-b593-d223f7449a82
50e9639f22