Use WP_Network_Query in ms_load_current_site_and_network

[spacedmonkey]

Use WP_Network_Query in ms_load_current_site_and_network instead of raw SQL query.

[spacedmonkey]

Replace raw sql to the sites table with wp_network_query. Number of briefs to this, wp_network_query has filters that are run. Also network queries also have caching out of the box.

[flixos90]

Let's keep the current-network cache in place for now, as it's less aggressively invalidated compared to the cache used by WP_Network_Query.

[flixos90]

41762.2.diff​ keeps the current-network cache key in place, and removes the now unused $wpdb global.

[flixos90]

In 41718:

Multisite: Use WP_Network_Query in ms_load_current_site_and_network().

This gets rid of the last readonly direct database query for networks in core. 🎉

Props spacedmonkey.
Fixes #41762.