Add release notes for 2.19.4 (#4375)

Author: opensearch-ci-bot

release-notes/opensearch-ml-commons.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
+
+### Bug Fixes
+* Fix CVE-2025-55163, CVE-2025-48924, CVE-2025-58057 ([#4339](https://github.com/opensearch-project/ml-commons/pull/4339))
+
+### Infrastructure
+* Onboard to s3 snapshots ([#4320](https://github.com/opensearch-project/ml-commons/pull/4320))
+* Pass isMultiTenancyEnabled across classes to early return index search ([#4113](https://github.com/opensearch-project/ml-commons/pull/4113))
+
+### Refactoring
+* Refactors undeploy models client with sdkClient bulk op ([#4077](https://github.com/opensearch-project/ml-commons/pull/4077))