Improve the user experience of manually finalizing batched background migrations

The following discussion from !129735 (merged) should be addressed:

• @stomlinson started a discussion:

  This command can be difficult for users to craft due to the nested escaping, and we already generate the correct command in an error message (https://gitlab.com/gitlab-org/gitlab/-/blob/master/lib/gitlab/database/migrations/batched_background_migration_helpers.rb#L241). Unrelated to this MR, but I think we should improve the UX here by giving people the command they need on the error page.