Adding release notes for 2.9.0 (#504)

Signed-off-by: Chenyang Ji <[email protected]>

release-notes/opensearch-performance-analyzer.release-notes-2.9.0.0.md

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+## Version 2.9.0 Release Notes
+
+Compatible with OpenSearch 2.9.0
+
+### Enhancements
+* Remove heap allocation rate as the input metric to HotShardClusterRca [#411](https://github.com/opensearch-project/performance-analyzer-rca/pull/411)
+* Set ThreadMetricsRca evaluation period from 12 seconds to 5 seconds [#410](https://github.com/opensearch-project/performance-analyzer-rca/pull/410)
+* Add unit tests for the REST layer in RCA Agent [#436](https://github.com/opensearch-project/performance-analyzer-rca/pull/436)
+
+### Bug fixes
+* Fix NPE issue in ShardStateCollector, which was impacted by changes from upstream core [#489](https://github.com/opensearch-project/performance-analyzer/pull/489)
+* Fix Mockito initialization issue [#443](https://github.com/opensearch-project/performance-analyzer-rca/pull/443)
+
+
+### Infrastructure
+* Update the BWC version to 2.8.0 [#446](https://github.com/opensearch-project/performance-analyzer/pull/446)
+* Upgrade bcprov to bcprov-jdk15to18 in performance-analyzer [#493](https://github.com/opensearch-project/performance-analyzer/pull/493)
+* Upgrade bcprov to bcprov-jdk15to18 in performance-analyzer-rca [439](https://github.com/opensearch-project/performance-analyzer-rca/pull/439)
+* Upgrade bcpkix to bcpkix-jdk15to18 in performance-analyzer-rca [446](https://github.com/opensearch-project/performance-analyzer-rca/pull/446)
+* Upgrade checkstyle version from 9.3 to 10.3.3 [#495](https://github.com/opensearch-project/performance-analyzer/pull/495)
+
+
+### Maintenance
+* Update build.gradle and github workflow to support 2.9 version [#499](https://github.com/opensearch-project/performance-analyzer/pull/499)
+* Update licenses files for 2.9 [#501](https://github.com/opensearch-project/performance-analyzer/pull/501)
+* Swap jboss annotation dependency for jakarta annotations [#407](https://github.com/opensearch-project/performance-analyzer-rca/pull/407)
+* Ensures compatibility check readiness [#438](https://github.com/opensearch-project/performance-analyzer-rca/pull/438)