Update CHANGELOG.md

Author: cristianoc

CHANGELOG.md

@@ -28,6 +28,8 @@
 - AST cleanup: Remove Pexp_function from the AST. https://github.com/rescript-lang/rescript/pull/7198
 - Remove unused code from Location and Rescript_cpp modules. https://github.com/rescript-lang/rescript/pull/7150
 - Build with OCaml 5.2.1. https://github.com/rescript-lang/rescript-compiler/pull/7201
+- AST cleanup: Remove `Function$` entirely for function definitions. https://github.com/rescript-lang/rescript/pull/7200
+
 
 # 12.0.0-alpha.5