nixos/modules: users.(extraUsers|extraGroup->users|group)

nixos/modules/services/databases/mysql.nix

@@ -221,13 +221,13 @@ in
       mkDefault (if versionAtLeast config.system.nixos.stateVersion "17.09" then "/var/lib/mysql"
                  else "/var/mysql");
 
-    users.extraUsers.mysql = {
+    users.users.mysql = {
       description = "MySQL server user";
       group = "mysql";
       uid = config.ids.uids.mysql;
     };
 
-    users.extraGroups.mysql.gid = config.ids.gids.mysql;
+    users.groups.mysql.gid = config.ids.gids.mysql;
 
     environment.systemPackages = [mysql];