Auth rule 1.3 doesn't permit the server to not know about room version 1

**Link to problem area**: https://spec.matrix.org/v1.6/rooms/v10/#authorization-rules

**Issue**
The condition on the rule is *if* the field is present, validate it. This doesn't *technically* allow the server to reject lack of a `room_version` field, which would only be the case in v1 rooms.

Servers are able to avoid implementing v1 if they don't want to. We should clarify that the auth rule does (realistically) require the `room_version` field to match the room version identifier.

We might want to move the generic check if "if you don't recognize the room version, bail" to the s2s spec rather than keep it within the room version - it doesn't really make sense here anyways.

Start with the linked Matrix specification section for room version v10 and inspect authorization rule 1.3 in context. Compare its wording with the S2S specification's handling of unknown room versions, then determine whether the generic check belongs there. Done means the specification unambiguously addresses missing or unrecognized room_version values and the scope is consistent across sections.

Written by the indexing model from the issue text.