Support `orderby => meta_value` in `WP_User_Query`. Adds unit test.

Props wpsmith.
Fixes #21581.


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Use `wp_get_nav_menus()` instead of `get_terms('nav_menu')` in `WP_Nav_Menu_Widget` to preserve use of `wp_get_nav_menus' filter.

Props Frank Klein.
Fixes #25263.


Built from https://develop.svn.wordpress.org/trunk@25332


git-svn-id: http://core.svn.wordpress.org/trunk@25294 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Remove dead code in `add_query_arg()`.

Props hakre, c3mdigital.
Fixes #16942.


Built from https://develop.svn.wordpress.org/trunk@25333


git-svn-id: http://core.svn.wordpress.org/trunk@25295 1a063a9b-81f0-0310-95a4-ce76da25c4cd