Adopt the Prefect best practices for an HA server when replicas > 1

[chrisguidry]

The documentation (early PR at https://github.com/PrefectHQ/prefect/pull/18279/files) discusses how to enable HA for Prefect servers if you are running PostgreSQL and Redis with a specific combination of API/UI and background services. Let's make this pattern easier to follow for a PrefectServer.

• We should have validation (validating webhook?) when enabling multiple replicas of PrefectServer that you're using PostgreSQL and providing a Redis connection
• We can allow users to request an additional background service deployment with its own independent replicas; this and the primary/API/UI deployment could each have the right command line options to run the right subset of services