Added release notes for OS-cross-cluster-replication_2.15.0 (#1399)

Signed-off-by: Mohit Kumar <mohitamg@amazon.com>
(cherry picked from commit 32f28cc)

Author: mohitamg

release-notes/opensearch-cross-cluster-replication.release-notes-2.15.0.0.md

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+## Version 2.15.0.0 Release Notes
+
+Compatible with OpenSearch 2.15.0
+
+### Maintenance
+* Disabling docrep to remote store migration test for integTestRemote suite ([#1379](https://github.com/opensearch-project/cross-cluster-replication/pull/1379))