nixos/dependency-track: fix default JVM heap size (#425747)

- `services.clamsmtp` is unmaintained and was removed from Nixpkgs.

- `services.dependency-track` removed its configuration of the JVM heap size. This lets the JVM choose its maximum heap size automatically, which should work much better in practice for most users. For deployments on systems with little RAM, it may now be necessary to manually configure a maximum heap size using  {option}`services.dependency-track.javaArgs`.

- `services.dnscrypt-proxy2` gains a `package` option to specify dnscrypt-proxy package to use.

- `services.gitea` supports sending notifications with sendmail again. To do this, activate the parameter `services.gitea.mailerUseSendmail` and configure SMTP server.

    javaArgs = lib.mkOption {

      type = lib.types.listOf lib.types.str;

      default = [ "-Xmx4G" ];

      description = "Java options passed to JVM";

      default = [ ];

      example = lib.literalExpression ''[ "-Xmx16G" ] '';

      description = ''

        Java options passed to JVM. Configuring this is usually not necessary, but for small systems

        it can be useful to tweak the JVM heap size.

      '';

    };

    database = {

        };

        services.dependency-track = {

          enable = true;

          # The Java VM defaults (correctly) to tiny heap on this tiny

          # VM, but that's not enough to start dependency-track.

          javaArgs = [ "-Xmx4G" ];

          port = dependencyTrackPort;

          nginx.domain = "localhost";

          database.passwordFile = "${pkgs.writeText "dbPassword" ''hunter2'THE'''H''''E''}";