Add release notes for 2.19.4

Signed-off-by: opensearch-ci <[email protected]>

Author: opensearch-ci-bot

release-notes/opensearch-index-management.release-notes-2.19.4.0.md

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+## Version 2.19.4 Release Notes
+
+Compatible with OpenSearch and OpenSearch Dashboards version 2.19.4
+
+### Enhancements
+* [Backport 2.19] Onboarding new maven snapshots publishing to s3 (IM) ([#1514](https://github.com/opensearch-project/index-management/pull/1514))
+
+### Maintenance
+* [AUTO] Increment version to 2.19.4-SNAPSHOT ([#1477](https://github.com/opensearch-project/index-management/pull/1477))
+* [Backport 2.19] Update the maven snapshot publish endpoint and credential ([#1481](https://github.com/opensearch-project/index-management/pull/1481))