Add Release Notes for v2.12.0 (#628) (#631)

Signed-off-by: Chenyang Ji <cyji@amazon.com>
(cherry picked from commit aabe211)

Co-authored-by: Chenyang Ji <cyji@amazon.com>

release-notes/opensearch-performance-analyzer.release-notes-2.12.0.0.md

@@ -0,0 +1,14 @@
+## Version 2.12.0 Release Notes
+
+Compatible with OpenSearch 2.12.0
+
+### Enhancements
+* Add separate metric for cluster manager service events and metrics [#579](https://github.com/opensearch-project/performance-analyzer/pull/579)
+
+### Infrastructure
+* Update to Gradle 8.5 and Fixing CVE-2023-33202 [#617](https://github.com/opensearch-project/performance-analyzer/pull/617)
+
+### Maintenance
+* Remove redundant ClusterManagerThrottlingMetricsCollector [#582](https://github.com/opensearch-project/performance-analyzer/pull/582)
+* Update spotless to meet JDK-21 baseline [#618](https://github.com/opensearch-project/performance-analyzer/pull/618)
+