Replace `is_super_admin()` check in `wp-admin/menu.php` with `update_core` capability check

[flixos90]

This is part of the #37616 task. update_core is a suitable capability to use in wp-admin/menu.php.

[jeremyfelt]

39060.diff​ looks good, @flixos90. I think it's okay to add the {} to the if clause during commit since we're updating that area anyway.

It does seem like we could revisit this area and clean up the logic at the top of the file a bit more, but that shouldn't be part of this change.

[flixos90]

In 39540:

Multisite: Replace is_super_admin() with update_core for update permissions.

Fixes #39060. See #37616.