add changelog entry

Author: GAtom22

tools/confix/CHANGELOG.md

@@ -31,6 +31,10 @@ Ref: https://keepachangelog.com/en/1.0.0/
 
 ## [Unreleased]
 
+### Features
+
+* (confix) [#21202](https://github.com/cosmos/cosmos-sdk/pull/21202) Allow customization of migration `PlanBuilder`.
+
 ## [v0.1.1](https://github.com/cosmos/cosmos-sdk/releases/tag/tools/confix/v0.1.1) - 2023-12-11
 
 * [#18496](https://github.com/cosmos/cosmos-sdk/pull/18496) Remove invalid non SDK config from app.toml migration templates.