nixos/postgresql: use a standard default value for dataDir

nixos/doc/manual/release-notes/rl-2009.xml

@@ -615,6 +615,17 @@ services.dokuwiki."mywiki" = {
   };
   ...
 };
+</programlisting>
+    </para>
+   </listitem>
+   <listitem>
+    <para>
+      The <xref linkend="opt-services.postgresql.dataDir"/> option is now set to <literal>"/var/lib/postgresql/${cfg.package.psqlSchema}"</literal> regardless of your
+      <xref linkend="opt-system.stateVersion"/>. Users with an existing postgresql install that have a <xref linkend="opt-system.stateVersion"/> of <literal>17.09</literal> or below
+      should double check what the value of their <xref linkend="opt-services.postgresql.dataDir"/> option is (<literal>/var/db/postgresql</literal>) and then explicitly
+      set this value to maintain compatibility:
+<programlisting>
+services.postgresql.dataDir = "/var/db/postgresql";
 </programlisting>
     </para>
    </listitem>

nixos/modules/services/databases/postgresql.nix

@@ -55,6 +55,7 @@ in
 
       dataDir = mkOption {
         type = types.path;
+        defaultText = "/var/lib/postgresql/\${config.services.postgresql.package.psqlSchema}";
         example = "/var/lib/postgresql/11";
         description = ''
           Data directory for PostgreSQL.
@@ -249,10 +250,7 @@ in
             else if versionAtLeast config.system.stateVersion "16.03" then pkgs.postgresql_9_5
             else throw "postgresql_9_4 was removed, please upgrade your postgresql version.");
 
-    services.postgresql.dataDir =
+    services.postgresql.dataDir = mkDefault "/var/lib/postgresql/${cfg.package.psqlSchema}";
-      mkDefault (if versionAtLeast config.system.stateVersion "17.09"
-                  then "/var/lib/postgresql/${cfg.package.psqlSchema}"
-                  else "/var/db/postgresql");
 
     services.postgresql.authentication = mkAfter
       ''