Adding change log for unreleased 2.x

Signed-off-by: Sagar Upadhyaya <sagar.upadhyaya.121@gmail.com>

CHANGELOG.md

@@ -86,6 +86,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 - Add descending order search optimization through reverse segment read. ([#7244](https://github.com/opensearch-project/OpenSearch/pull/7244))
 - Add 'unsigned_long' numeric field type ([#6237](https://github.com/opensearch-project/OpenSearch/pull/6237))
 - Add back primary shard preference for queries ([#7375](https://github.com/opensearch-project/OpenSearch/pull/7375))
+- Adding task cancellation timestamp in task API ([#7455](https://github.com/opensearch-project/OpenSearch/pull/7455))
 
 ### Dependencies
 - Bump `com.netflix.nebula:gradle-info-plugin` from 12.0.0 to 12.1.0