Adding issue to CHANGELOG.md

Author: Rakshit Goyal

CHANGELOG.md

@@ -24,6 +24,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 - Decouple the init of Crypto Plugin and KeyProvider in CryptoRegistry ([18270](https://github.com/opensearch-project/OpenSearch/pull18270)))
 
 ### Changed
+- Reject Close index requests, While DocRep to SegRep migration is in progress.([#18327](https://github.com/opensearch-project/OpenSearch/pull/18327))
 
 ### Dependencies
 - Bump `com.google.code.gson:gson` from 2.12.1 to 2.13.1 ([#17923](https://github.com/opensearch-project/OpenSearch/pull/17923), [#18266](https://github.com/opensearch-project/OpenSearch/pull/18266))