Bump version to 2.9

Bump version from 2.8 to 2.9

Issue:
opensearch-project#4074

Signed-off-by: Kawika Avilla <kavilla414@gmail.com>

Author: kavilla

package.json

@@ -11,7 +11,7 @@
     "dashboarding"
   ],
   "private": true,
-  "version": "2.8.0",
+  "version": "2.9.0",
   "branch": "2.x",
   "types": "./opensearch_dashboards.d.ts",
   "tsdocMetadata": "./build/tsdoc-metadata.json",