* 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
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