Add 0.21.0 to CHANGELOG

Ref. eng/recordflux/RecordFlux#1594

CHANGELOG.md

@@ -5,11 +5,11 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
-## [Unreleased]
+## [0.21.0] - 2024-04-23
 
 ### Changed
 
-- Improve error messages for type refinements of non-message types (eng/recordflux/RecordFlux#383)
+- Improve error messages for type refinements of non-message types (AdaCore/RecordFlux#383, eng/recordflux/RecordFlux#383)
 
 ### Fixed
 
@@ -506,7 +506,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [0.1.0] - 2019-05-14
 
-[Unreleased]: https://github.com/AdaCore/RecordFlux/compare/v0.20.0...HEAD
+[0.21.0]: https://github.com/AdaCore/RecordFlux/compare/v0.20.0...v0.21.0
 [0.20.0]: https://github.com/AdaCore/RecordFlux/compare/v0.19.0...0.20.0
 [0.19.0]: https://github.com/AdaCore/RecordFlux/compare/v0.18.0...v0.19.0
 [0.18.0]: https://github.com/AdaCore/RecordFlux/compare/v0.17.0...v0.18.0