Add async index to prepared_at column for merge requests

What does this MR do and why?

In the MR !121999 (merged), we want to add a index to prepared_at column on Merge Requests.

However, the migration is quiet long, so let's do it async.

Follow up issue: #415721 (closed)

Async Docs: https://docs.gitlab.com/ee/development/database/adding_database_indexes.html#create-indexes-asynchronously