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.
Merges [43001] to the 4.9 branch.
Fixes#42202.
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