update changelog

Author: pinin4fjords

CHANGELOG.md

@@ -9,6 +9,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Enhancements and fixes
 
+- [PR #1608](https://github.com/nf-core/rnaseq/pull/1608) - Bump version after release 3.21.0
+
 ## [[3.21.0](https://github.com/nf-core/rnaseq/releases/tag/3.21.0)] - 2025-09-18
 
 ### Credits