Create opensearch-cross-cluster-replication.release-notes-2.12.0.0.md (#1321) (#1323)

Add Release notes for OS 2.12.0

Signed-off-by: Mohit Kumar <113413713+mohitamg@users.noreply.github.com>
(cherry picked from commit 720e080)

Co-authored-by: Mohit Kumar <113413713+mohitamg@users.noreply.github.com>

Author: opensearch-trigger-bot[bot]

release-notes/opensearch-cross-cluster-replication.release-notes-2.12.0.0.md

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+## Version 2.12.0 Release Notes
+
+Compatible with OpenSearch 2.12.0
+
+## Bug Fixes
+
+* Implement getSystemIndexDescriptors to support SystemIndex for replication plugin ([#1290](https://github.com/opensearch-project/cross-cluster-replication/pull/1290))
+* Correct error message including what fields are missing when field are not passed when starting replication ([#1292](https://github.com/opensearch-project/cross-cluster-replication/pull/1292))
+* Ignoring all the final settings to copy from leader to follower as those settings won't be able to apply as those are not updatable ([#1304](https://github.com/opensearch-project/cross-cluster-replication/pull/1304))