[Backport 2.9] Create 2.9 release notes and clear out changelog (#8682)#8685

Merged

reta merged 1 commit intoopensearch-project:2.9from andrross:backport/backport-8682-to-2.9

Jul 13, 2023

+111-112

Commits on Jul 13, 2023

Create 2.9 release notes and clear out changelog (opensearch-project#8682 )
andrross
committed