nixos/stalwart-mail: pin module to package version 0.6

- `buildDubPackage` can now be used to build Programs written in [D](https://dlang.org/) using the `dub` build system and package manager.

  See the [D section](https://nixos.org/manual/nixpkgs/unstable#dlang) in the manual for more information.

- `stalwart-mail` service uses the legacy version 0.6.X as default because newer `stalwart-mail` versions require a [manual upgrade process](https://github.com/stalwartlabs/mail-server/blob/main/UPGRADING.md). Change [`services.stalwart-mail.package`](#opt-services.stalwart-mail.package) to `pkgs.stalwart-mail` if you wish to switch to the new version.

- [`portunus`](https://github.com/majewsky/portunus) has been updated to major version 2.

  This version of Portunus supports strong password hashes, but the legacy hash SHA-256 is also still supported to ensure a smooth migration of existing user accounts.

  After upgrading, follow the instructions on the [upstream release notes](https://github.com/majewsky/portunus/releases/tag/v2.0.0) to upgrade all user accounts to strong password hashes.

  configFormat = pkgs.formats.toml { };

  configFile = configFormat.generate "stalwart-mail.toml" cfg.settings;

  dataDir = "/var/lib/stalwart-mail";

  stalwartAtLeast = versionAtLeast cfg.package.version;

in {

  options.services.stalwart-mail = {

    enable = mkEnableOption "the Stalwart all-in-one email server";

    package = mkPackageOption pkgs "stalwart-mail" { };

    package = mkOption {

      type = types.package;

      description = ''

        Which package to use for the Stalwart mail server.

        ::: {.note}

        Upgrading from version 0.6.0 to version 0.7.0 or higher requires manual

        intervention. See <https://github.com/stalwartlabs/mail-server/blob/main/UPGRADING.md>

        for upgrade instructions.

        :::

      '';

      default = pkgs.stalwart-mail_0_6;

      defaultText = lib.literalExpression "pkgs.stalwart-mail_0_6";

      example = lib.literalExpression "pkgs.stalwart-mail";

      relatedPackages = [ "stalwart-mail_0_6" "stalwart-mail" ];

    };

    settings = mkOption {

      inherit (configFormat) type;

  };

  config = mkIf cfg.enable {

    warnings = lib.optionals (!stalwartAtLeast "0.7.0") [

      ''

        Versions of stalwart-mail < 0.7.0 will get deprecated in NixOS 24.11.

        Please set services.stalwart-mail.package to pkgs.stalwart-mail to

        upgrade to the latest version.

        Please note that upgrading to version >= 0.7 requires manual

        intervention, see <https://github.com/stalwartlabs/mail-server/blob/main/UPGRADING.md>

        for upgrade instructions.

      ''

    ];

    # Default config: all local

    services.stalwart-mail.settings = {

      global.tracing.method = mkDefault "stdout";