Add the sync index creation
What does this MR do and why?
Following on from the creation of the Async prepared_at index, we need to now add it concurrently in order to update the schema, as per: https://docs.gitlab.com/ee/development/database/adding_database_indexes.html#create-indexes-asynchronously
Async Index MR: !124117 (merged)
Related MR that requires prepared_at index: !121999 (merged)
Index verification: https://console.postgres.ai/gitlab/gitlab-production-tunnel-pg12/sessions/20367/commands/66507
Edited by Marc Shaw