message: [docs] Advise about celery 4.3.0 requirement on redis
diff: @@ -23,6 +23,10 @@ assists people when migrating to a new version.
  ## Next Version
  +* [7848](https://github.com/apache/incubator-superset/pull/7848): If you are
  +running redis with celery, celery bump to 4.3.0 requires redis-py upgrade to
  +3.2.0 or later.
  +
  * [7667](https://github.com/apache/incubator-superset/pull/7667): a change to
  make all Unix timestamp (which by definition are in UTC) comparisons refer
  to a timestamp in UTC as opposed to local time.
  