Fix sliding sync for rooms with unknown room version

[erikjohnston]

Follow on from #17727

Example stack trace:

        Traceback (most recent call last):
          File "/synapse/synapse/http/server.py", line 332, in _async_render_wrapper
            callback_return = await self._async_render(request)
          File "/synapse/synapse/http/server.py", line 544, in _async_render
            callback_return = await raw_callback_return
          File "/synapse/synapse/rest/client/sync.py", line 940, in on_POST
            sliding_sync_results = await self.sliding_sync_handler.wait_for_sync_for_user(
          File "/synapse/synapse/handlers/sliding_sync/__init__.py", line 151, in wait_for_sync_for_user
            result = await self.current_sync_for_user(
          File "/synapse/synapse/logging/opentracing.py", line 922, in _wrapper
            return await func(*args, **kwargs)
          File "/synapse/synapse/handlers/sliding_sync/__init__.py", line 228, in current_sync_for_user
            interested_rooms = await self.room_lists.compute_interested_rooms(
          File "/synapse/synapse/handlers/sliding_sync/room_lists.py", line 186, in compute_interested_rooms
            return await self._compute_interested_rooms_new_tables(
          File "/synapse/synapse/logging/opentracing.py", line 922, in _wrapper
            return await func(*args, **kwargs)
          File "/synapse/synapse/handlers/sliding_sync/room_lists.py", line 383, in _compute_interested_rooms_new_tables
            sorted_room_info = await self.sort_rooms(
          File "/synapse/synapse/logging/opentracing.py", line 922, in _wrapper
            return await func(*args, **kwargs)
          File "/synapse/synapse/handlers/sliding_sync/room_lists.py", line 1922, in sort_rooms
            return sorted(
          File "/synapse/synapse/handlers/sliding_sync/room_lists.py", line 1925, in <lambda>
            key=lambda room_info: last_activity_in_room_map[room_info.room_id],
        KeyError: '!bkCPfqjchccmVTmolv:test'

[erikjohnston]

cc @kegsay

[kegsay]

Seems plausible.