Added release notes for 1.3.20 (opensearch-project#16824)

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

Author: bshien

release-notes/opensearch.release-notes-1.3.20.md

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+## 2024-12-10 Version 1.3.20 Release Notes
+
+### Dependencies
+- Bump `icu4j` from 62.1 to 62.2 ([#15469](https://github.com/opensearch-project/OpenSearch/pull/15469))
+- Bump `org.bouncycastle:bc-fips` from 1.0.2.4 to 1.0.2.5 ([#13446](https://github.com/opensearch-project/OpenSearch/pull/13446))
+- Bump `Netty` from 4.1.112.Final to 4.1.115.Final ([#16661](https://github.com/opensearch-project/OpenSearch/pull/16661))
+- Bump `avro` from 1.11.3 to 1.11.4 ([#16773](https://github.com/opensearch-project/OpenSearch/pull/16773))
+- Bump `commonsio` to 2.16.0 ([#16780](https://github.com/opensearch-project/OpenSearch/pull/16780))
+- Bump `protobuf-java` to 3.25.5 ([#16792](https://github.com/opensearch-project/OpenSearch/pull/16792))
+- Bump `snappy-java` to 1.1.10.7 ([#16792](https://github.com/opensearch-project/OpenSearch/pull/16792))
+
+### Fixed
+- Update help output for _cat ([#14722](https://github.com/opensearch-project/OpenSearch/pull/14722))
+- Bugfix to guard against stack overflow errors caused by very large reg-ex input ([#16101](https://github.com/opensearch-project/OpenSearch/pull/16101))