Update branch name to main (opendistro-for-elasticsearch#36)

Since main is default branch for cli,
update release draft to run after merge on main.

Author: VijayanB

.github/workflows/draft-release-notes-workflow.yml

@@ -3,7 +3,7 @@ name: Release Drafter
 on:
   push:
     branches:
-      - master
+      - main
 
 jobs:
   update_release_draft: