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

pkgs/servers/sql/postgresql/default.nix

@@ -182,7 +182,7 @@ in self: {
 
   postgresql_10 = self.callPackage generic {
     version = "10.6";
-    psqlSchema = "10.0";
+    psqlSchema = "10.0"; # should be 10, but changing it is invasive
     sha256 = "0jv26y3f10svrjxzsgqxg956c86b664azyk2wppzpa5x11pjga38";
     this = self.postgresql_10;
     inherit self;
@@ -190,7 +190,7 @@ in self: {
 
   postgresql_11 = self.callPackage generic {
     version = "11.2";
-    psqlSchema = "11.2";
+    psqlSchema = "11.1"; # should be 11, but changing it is invasive
     sha256 = "01clq2lw0v83zh5dc89xdr3mmap0jr37kdkh401ph6f2177bjxi6";
     this = self.postgresql_11;
     inherit self;