Update changelog for qualimap optimization PR

Add changelog entry for PR #1621 which optimizes qualimap performance
through multi-threaded name sorting.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

Author: pinin4fjords

CHANGELOG.md

@@ -15,6 +15,7 @@ Special thanks to the following for their contributions to the release:
 
 - [PR #1608](https://github.com/nf-core/rnaseq/pull/1608) - Bump version after release 3.21.0
 - [PR #1617](https://github.com/nf-core/rnaseq/pull/1617) - Update bbmap/bbsplit module
+- [PR #1621](https://github.com/nf-core/rnaseq/pull/1621) - Optimize qualimap performance with multi-threaded name sorting
 
 ## [[3.21.0](https://github.com/nf-core/rnaseq/releases/tag/3.21.0)] - 2025-09-18