Cleanup PostgreSQL for state version 17.09 (#25753)

* postgresql service: make 9.6 the default version for 17.09

* postgresql service: change default superuser for 17.09

Change the default superuser from `root` to `postgres` for state
version 17.09

* postgresql service: change default data directory for 17.09

The new directory includes the schema version of the database.
This makes upgrades easier and is more consistent with other distros.

* updated nixos release notes