docs: update changelog

Author: Yuri Zmytrakov

CHANGELOG.md

@@ -14,6 +14,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 ### Fixed
 
 - Fixed datetime filtering for .0Z milliseconds to preserve precision in apply_filter_datetime, ensuring only items exactly within the specified range are returned. [#535](https://github.com/stac-utils/stac-fastapi-elasticsearch-opensearch/pull/535)
+- Normalize datetime in POST /search requests to match GET /search behavior. [#543](https://github.com/stac-utils/stac-fastapi-elasticsearch-opensearch/pull/543)
 
 ### Removed