Moving these functions to `wp-includes/capabilities.php` allows plugins to access more than just `is_super_admin()` and `get_super_admin()` from the front end without including a file via `wp-admin/`. Add an `is_multisite()` check to prevent use of these on single site environments now that they have left `ms.php`..
Props johnjamesjacoby.
Fixes#21788.
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