chore: update changelog

CHANGELOG.md

@@ -15,6 +15,11 @@ Types of changes:
 
 ## Unreleased
 
+## [1.2.0-rc1] - 2023-08-24
+### Changed
+- Clarified that UUIDs for attachments are globally unique. (planning#1134, 524ca8b)
+
+## [1.2.0-rc0] - 2023-08-23
 ### Added
 
 - Add `replyChannels` attribute. Using this attribute, an Onlineservice (Sender) can tell the Fachverfahren (Subscriber)