Copy room `type` on upgrade too (#1198)

MSC: https://github.com/matrix-org/matrix-spec-proposals/pull/3818

changelogs/client_server/newsfragments/1198.feature

@@ -0,0 +1 @@
+Copy a room's `type` when upgrading it, as per [MSC3818](https://github.com/matrix-org/matrix-spec-proposals/pull/3818).

content/client-server-api/modules/room_upgrades.md

@@ -33,8 +33,11 @@ server:
 1.  Checks that the user has permission to send `m.room.tombstone`
     events in the room.
 
-2.  Creates a replacement room with a `m.room.create` event containing a
-    `predecessor` field and the applicable `room_version`.
+2.  {{< changed-in v="1.4" >}} Creates a replacement room with a `m.room.create` event containing a
+    `predecessor` field, the applicable `room_version`, and a `type` field
+    which is copied from the `predecessor` room. If no `type` is set on the
+    previous room, no `type` is specified on the new room's create event
+    either.
 
 3.  Replicates transferable state events to the new room. The exact
     details for what is transferred is left as an implementation detail,