add 2.16 release note (#1268) (#1269)

(cherry picked from commit eb7aad4)

Signed-off-by: Jackie Han <jkhanjob@gmail.com>
Signed-off-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>
Co-authored-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>

release-notes/opensearch-anomaly-detection.release-notes-2.16.0.0.md

@@ -0,0 +1,18 @@
+## Version 2.16.0.0 Release Notes
+
+Compatible with OpenSearch 2.16.0
+
+### Feature
+* Adding support for date_nanos to Anomaly Detection ([#1238](https://github.com/opensearch-project/anomaly-detection/pull/1238))
+
+### Enhancements
+
+* update BWC test version and enhance code coverage([#1253](https://github.com/opensearch-project/anomaly-detection/pull/1253))
+* Register system index descriptors through SystemIndexPlugin.getSystemIndexDescriptors([#1251](https://github.com/opensearch-project/anomaly-detection/pull/1251))
+* Add feature filtering in model validation ([#1258](https://github.com/opensearch-project/anomaly-detection/pull/1258))
+
+### Infrastructure
+* set baseline JDK version to JDK-21 ([#1228](https://github.com/opensearch-project/anomaly-detection/pull/1228))
+
+### Maintenance
+* Update PULL_REQUEST_TEMPLATE to include an API spec change in the checklist ([#1262](https://github.com/opensearch-project/anomaly-detection/pull/1262))