Add release notes for release 1.3.19 (opensearch-project#15392)

Signed-off-by: Brandon Shien <bshien@amazon.com>

Author: bshien

release-notes/opensearch.release-notes-1.3.19.md

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+## 2024-08-22 Version 1.3.19 Release Notes
+
+### Upgrades
+- OpenJDK Update (July 2024 Patch releases) ([#15002](https://github.com/opensearch-project/OpenSearch/pull/15002))
+- Bump `netty` from 4.1.111.Final to 4.1.112.Final ([#15081](https://github.com/opensearch-project/OpenSearch/pull/15081))