message: docs: Add potential downtime line item to UPDATING.md
diff: @@ -39,6 +39,8 @@ assists people when migrating to a new version.
  ### Potential Downtime
  +- [21284](https://github.com/apache/superset/pull/21284): A change which drops the unused `dbs.allow_multi_schema_metadata_fetch` column via a (potentially locking) DDL operation.
  +
  ### Other
  ## 2.0.0
  