Stabilize support for MSC3288: room type on the store-invite API

[clokep]

Once MSC3288 lands we should (at some point) switch to a stable identifier.

There's a single case to switch:

synapse/synapse/handlers/identity.py

Lines 879 to 880 in eb9ddc8

	if room_type is not None:
	invite_config["org.matrix.msc3288.room_type"] = room_type

We might want to send both the stable and unstable identifiers for a bit.

[clokep]

Note that Synapse v1.47.0 (released on 2021-11-17) started sending both the unstable and stable identifier. I think we can drop it now.