chore: changelog v1.2.1 (#3394)

* chore: changelog v1.2.1

Rename the v1.2.0 section to v1.2.1 with a note that v1.2.0 was
superseded (it was tagged before core/v1.1.0 existed and required an
unavailable core version). The GitHub Release workflow reads
CHANGELOG.md by version, so the release tag needs a matching section.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

* Apply suggestion from @tac0turtle

---------

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

Author: tac0turtle

CHANGELOG.md

@@ -9,7 +9,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
-## v1.2.0
+## v1.2.1
+
 
 ### Added