Add "No significant changes" to changelog

CHANGES.md

@@ -3,6 +3,8 @@ Synapse 1.42.0 (2021-09-07)
 This version of Synapse removes deprecated room-management admin APIs, removes out-of-date email pushers, and improves error handling for fallback templates for user-interactive authentication.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).
+No significant changes since 1.42.0rc2.
+
 Synapse 1.42.0rc2 (2021-09-06)
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