Use `promote_users` for role updates in edit_user()

[ocean90]

edit_user() can also update user roles but is still using the edit_users capability instead of the newer promote_users capability introduced in [14176].

We have a similar role handling in https://core.trac.wordpress.org/browser/tags/4.8.3/src/wp-admin/users.php?marks=96-97,121-128#L93 and should copy that to edit_user() for consistency.

• Change edit_users to promote_users
• Change manage_site to manage_network_users

[ocean90]

Also props to @jjj and @flixos90

[ocean90]

In 42855:

Users: Use promote_users for role updates in edit_user().

edit_user() can also update user roles but was still using the edit_users capability instead of the newer promote_users capability introduced in [14176].
This makes the role handling consistent with the bulk dropdown menu for role changes.

Props flixos90, johnjamesjacoby, ocean90.
Fixes #42564.