Rename 'populate_stats_process_rooms_2' background job back to 'populate_stats_process_rooms' again (#8243)

Fixes https://github.com/matrix-org/synapse/issues/8238

Alongside the delta file, some changes were also necessary to the codebase to remove references to the now defunct `populate_stats_process_rooms_2` background job. Thankfully the latter doesn't seem to have made it into any documentation yet :)