Postgres: no warning when migrations are not up to date

[williamdclt]

If a new version of SpiceDB (with a Postgres datastore) is deployed that contains migrations, the datastore needs to be migrated before rolling out the new version (eg with a pre-upgrade hook on Kubernetes).

If this isn't done, the pods report as unhealthy and we see the error {"level":"debug","datastoreReady":false,"dispatchReady":true,"time":"2022-08-08T12:20:53Z","message":"completed dispatcher and datastore readiness checks"}. This error does not mention migrations at all, it makes it hard to understand what is going wrong: it would be great to get a log (I'd expect level warn) that indicates that there are migrations pending. Possibly it would be useful to make it part of the health check response.