nixos/dovecot: Fix usage of dhparams option

The pull request that added dhparams (#39507) was made at the time where
the dhparams module overhaul (#39526) wasn't done yet, so it's still
using the old mechanics of the module.

As stated in the release notes:

  Module implementers should not set a specific bit size in order to let
  users configure it by themselves if they want to have a different bit
  size than the default (2048).

  An example usage of this would be:

    { config, ... }:

    {
      security.dhparams.params.myservice = {};
      environment.etc."myservice.conf".text = ''
        dhparams = ${config.security.dhparams.params.myservice.path}
      '';
    }

Signed-off-by: aszlig <aszlig@nix.build>
Cc: @qknight, @abbradar, @hrdinka, @leenaars