update changelog

Signed-off-by: Nicholas Walter Knize <[email protected]>

Author: nknize

CHANGELOG.md

@@ -102,6 +102,7 @@ Inspired from [Keep a Changelog](https://keepachangelog.com/en/1.0.0/)
 - Always auto release the flood stage block ([#4703](https://github.com/opensearch-project/OpenSearch/pull/4703))
 - Remove LegacyESVersion.V_7_4_ and V_7_5_ Constants ([#4704](https://github.com/opensearch-project/OpenSearch/pull/4704))
 - Remove Legacy Version support from Snapshot/Restore Service ([#4728](https://github.com/opensearch-project/OpenSearch/pull/4728))
+- Remove deprecated serialization logic from pipeline aggs ([#4847](https://github.com/opensearch-project/OpenSearch/pull/4847))
 
 ### Fixed
 - `opensearch-service.bat start` and `opensearch-service.bat manager` failing to run ([#4289](https://github.com/opensearch-project/OpenSearch/pull/4289))