Support v2 of the room delete API

[JOJ0]

and make sure pre-1.48 Synapse is still using delete API v1: https://matrix-org.github.io/synapse/latest/admin_api/rooms.html#delete-room-api

Also the new v2 API changed some behaviour:

block - Optional. If set to true, this room will be added to a blocking list, preventing future attempts to join the room. Rooms can be blocked even if they're not yet known to the homeserver (only with Version 1 of the API). Defaults to false.