event_queue: Update rule for when we can remove compatibility code.

Since c3a8a15bae removed the last
instance of code using the dictionary code path, we actually need to
wait until one can no longer upgrade directly from 4.x to master in
order to avoid breakage should we remove this compatibility code,
since only today did we stop generating the old event format.

zerver/tornado/event_queue.py

@@ -1207,7 +1207,7 @@ def process_notification(notice: Mapping[str, Any]) -> None:
             # queue at the time of upgrade.
             #
             # TODO/compatibility: Remove this block once you can no
-            # longer directly upgrade directly from 3.x to master.
+            # longer directly upgrade directly from 4.x to master.
             user_ids: List[int] = [user["id"] for user in cast(List[Mapping[str, int]], users)]
         else:
             user_ids = cast(List[int], users)