Don't use deprecated WP_Screen::is_network property.

[bpetty]

Just updates WP_Plugins_List_Table to use $screen->in_admin( 'network' ) instead of the deprecated is_network property.

is_network was deprecated in r21687 (#21742), and while this might normally use in_network_admin(), it's possible to pass in a custom WP_Screen into WP_List_Table (which might happen in unit tests or maybe some ajax calls), so in this case, the replacement continues to use the same screen object instead.

[SergeyBiryukov]

In 23658:

Replace deprecated WP_Screen::is_network property with WP_Screen::in_admin( 'network' ). props bpetty. fixes #23215.