Support room version 10

[richvdh]

https://spec.matrix.org/latest/rooms/v10/ is a thing. I think the changes are:

• support the knock_restricted join rule. We already have this in the MSC3787 room version type, so this should be trivial
• forbid strings in PL events, per https://spec.matrix.org/latest/rooms/v10/#values-in-mroompower_levels-events-must-be-integers

[clokep]

forbid strings in PL events, per spec.matrix.org/latest/rooms/v10/#values-in-mroompower_levels-events-must-be-integers

I think this is partially done in #11885.

[turt2live]

#12805 is the very terribly written PR for v10 - please feel free to take it over (or ignore it and write it properly)