Administration: Update `WP_List_Table::get_sortable_columns()` to support...

Administration: Update `WP_List_Table::get_sortable_columns()` to support `asc` and `desc` arguments.

This makes the API a little more clear, whereas setting `false` used to mean `asc` and `true` meant `desc`, you can now use those directly, while maintaining back-compat.

Fixes #45089.

Props Tkama, SergeyBiryukov, shital-patel, desrosj, birgire, davidbaumwald. 


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