postgresql: fix regression in NixOS after update to 11.2 (#56146)

ryantm bot updates `psqlSchema` alongside with version: https://github.com/NixOS/nixpkgs/pull/50646, https://github.com/NixOS/nixpkgs/pull/55860
but `psqlSchema` is used to generate `stateDir`. It shouldn't be updated.

Ideally it should be equal to postgresql major version for pg versions,
but we already have deployments in /var/lib/postgresql/11.1. Strange why
nobody complained when it was changed from 11.0 -> 11.1.

I propose do correct naming for PG12+, but status quo for current versions.

Fixes https://github.com/NixOS/nixpkgs/issues/56134