Fix Praefect config file to handle virtual storage changes

As discussed in gitaly#2118 (closed) and implemented in gitaly!1606 (merged), Praefect now supports multiple virtual storages, and the root-level virtual_storage_name is now deprecated (https://gitlab.com/gitlab-org/gitaly/blob/29a9cee842181c28111c166d9741f06abf06a829/internal/praefect/config/config.go#L22-23).

This was needed to get replication working locally for me.