We use go-migrate to handle migrations. Only the "up" direction is
done for now. As migrations are handled inside the application,
writing downgrades is not really useful as it would be needed when
downgrading an app, but then we cannot downgrade. Moreover, the way
the flow topics are versioned, you can downgrade without much dommage.
We need to version flow schemas. Otherwise, this won't be manageable.
Confluent is pushing for a registry, but it seems the ecosystem is
still too young. Let's version on our side with a topic for each
version.
