Merge branch 'am-add-ff-defintion-database_async_index_creation' into 'master'

Add definition for database_async_index_creation ff

See merge request gitlab-org/gitlab!76351

config/feature_flags/ops/database_async_index_creation.yml

+---
+name: database_async_index_creation
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/66478
+rollout_issue_url:
+milestone: '14.2'
+type: ops
+group: group::database
+default_enabled: false