Update CHANGELOG.md for version 0.6.21

Author: actions-user

CHANGELOG.md

@@ -13,6 +13,17 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Fixed
 
+### Removed
+
+
+## [0.6.21] - 2025-04-25
+
+### Added
+
+### Changed
+
+### Fixed
+
 - Fixed error "'UtcDatetime' object has no attribute 'timestamp'" in step trace:
   - Modified `_add_step_trace` method in `wish-command-execution/backend/bash.py` to use existing `__sub__` method
   - Used `timedelta.total_seconds()` to calculate execution time instead of direct timestamp access