Move the upgrade notes reminder up to rc2

CHANGES.md

 Synapse 1.42.0rc2 (2021-09-06)
 ==============================
+Server administrators are reminded to read [the upgrade notes](docs/upgrade.md#upgrading-to-v1420).
+
 Features
 --------
@@ -16,9 +18,6 @@ Internal Changes
 Synapse 1.42.0rc1 (2021-09-01)
 ==============================
-Server administrators are reminded to read [the upgrade notes](docs/upgrade.md#upgrading-to-v1420).
-
-
 Features
 --------