Add 2.6 release notes

Signed-off-by: Tyler Ohlsen <ohltyler@amazon.com>

release-notes/opensearch-anomaly-detection-dashboards.release-notes-2.6.0.0.md

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+## Version 2.6.0.0 Release Notes
+
+Compatible with OpenSearch Dashboards 2.6.0
+
+### Enhancements
+
+* Update cold start message ([#398](https://github.com/opensearch-project/anomaly-detection-dashboards-plugin/pull/398))
+* Changed required minimum intervals in cold start message ([#411](https://github.com/opensearch-project/anomaly-detection-dashboards-plugin/pull/411))
+* Remove `auto_expand_replicas` override in sample data indices ([#423](https://github.com/opensearch-project/anomaly-detection-dashboards-plugin/pull/423))
+
+### Bug Fixes
+
+* upgrade filter bug ([#402](https://github.com/opensearch-project/anomaly-detection-dashboards-plugin/pull/402))
+
+### Infrastructure
+
+* Bump @sideway/formula to 3.0.1 ([#418](https://github.com/opensearch-project/anomaly-detection-dashboards-plugin/pull/418))