Remove PostresSingleServer and improve deployer reliability

[BMurri]

fixes #743
Requires microsoft/ga4gh-tes#552 as a prerequisite.

[jsaun]

I would prefer to wait to remove single server until private link for flexible server is GA. https://learn.microsoft.com/en-us/azure/postgresql/flexible-server/concepts-networking-private-link . Providence is still using single server and won't switch to flexible until the private networking is GA.

[jsaun]

That said, Providence is mostly a custom deployment already so I'm sure they could continue using single server regardless of what we do in our deployer, but it does make it that much harder to replicate their deployment ourselves.

[BMurri]

This doesn't force a switch. Current deployments with single serve should still upgrade correctly without changing/removing/dropping support for them. It just won't enable new deployments with single server Postgres.

[jsaun]

We could also have another organization with similar networking requirements and azure policies around private preview features want to deploy CoA, but I suppose we can always point them to an older version.. Its probably fine, I'm a just a bit more risk adverse in terms of removing functionality.