Document manual migration of `unicorn` settings to `puma`

We need to ensure we clearly document that unicorn settings will not be automatically migrated over to puma. The best place for this is likely the upgrade notes, and then to make a reference from the Puma docs for now until we are past the migration period.

For reference, the upgrade notes I am referring to are like: https://docs.gitlab.com/omnibus/update/gitlab_12_changes.html.