Move upgrade notice up in changelog

CHANGES.md

 Synapse 1.42.0 (2021-09-07)
 ===========================
-No significant changes.
+This version of Synapse removes deprecated room-management admin APIs, removes out-of-date email pushers, and improves error handling for fallback templates for user-interactive authentication. For more information on these points, server administrators are encouraged to read [the upgrade notes](docs/upgrade.md#upgrading-to-v1420).
 Synapse 1.42.0rc2 (2021-09-06)
 ==============================
-This version of Synapse removes deprecated room-management admin APIs, removes out-of-date
-email pushers, and improves error handling for fallback templates for user-interactive
-authentication. For more information on these points, server administrators are
-encouraged to read [the upgrade notes](docs/upgrade.md#upgrading-to-v1420).
-
 Features
 --------