Unblacklist /upgrade creates a new room (#7228)

This commit was originally merged in #7228 but reverted in #7254 as the
associated sytest was not ready yet. Now that it is, we can merge this
again.

.buildkite/worker-blacklist

@@ -5,8 +5,6 @@ Message history can be paginated
 
 Can re-join room if re-invited
 
-/upgrade creates a new room
-
 The only membership state included in an initial sync is for all the senders in the timeline
 
 Local device key changes get to remote servers

changelog.d/7228.misc

@@ -0,0 +1 @@
+Unblacklist '/upgrade creates a new room' sytest for workers.