Replace `is_super_admin()` check when setting up multisite

[flixos90]

When setting up multisite, an is_super_admin() check is made in populate_network(). As discussed with @johnbillion at WordCamp Torino, that check can easily be removed, and instead the previous query can be adjusted to query users with the administrator role only.

This ticket is part of the #37616 task and is described in https://core.trac.wordpress.org/ticket/37616#comment:62.

[flixos90]

In 40406:

Multisite: Replace unnecessary is_super_admin() check when setting up the initial network.

When using is_super_admin() in a non-multisite environment, the function is supposed to check for administrator capabilities. The process of querying all users and filtering them with that function can be optimized by only querying users with the administrator role instead.

Fixes #40406. See #37616.