nixos/mattermost: require Postgres for version 11+

            or hostname, and services.mattermost.port to specify the port separately.

          '';

        }

        {

          # Can't use MySQL on version 11.

          assertion = versionAtLeast cfg.package.version "11" -> cfg.database.driver == "postgres";

          message = ''

            Only Postgres is supported as the database driver in Mattermost 11 and later.

          '';

        }

      ];

    })

    (mkIf cfg.matterircd.enable {