When updating room_stats_state, we try to check for null bytes slipping
in to the
content for state events. It turns out we had added guest_access as a
field to
room_stats_state without including it in the null byte check.
Lo and behold, a null byte in a m.room.guest_access event then breaks
room_stats_state
updates.
This PR adds the check for guest_access. A further PR will improve this
function so that this hopefully does not happen again in future.
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 :)
