[FEATURE] - Not actually require postfix, or a way to override that

## User Story

As a homelab server runner, I want to not be required to install postfix, because I'm using a different `sendmail` provider.

I've got `msmtp` installed, which relays to my node that runs actual postfix to do heavier SMTP bits. having postfix installed also conflicts with this, and doesn't need to be there.

## Basic info

* **Distro:** Ubuntu 20.04
* **Game:** Valheim (in this case, but it'd apply to anything I think)
* **Command:** check-update
* **LinuxGSM version:** v21.2.3

## Further Information

It'd be nice if it would detect that it's got a `sendmail` command to do the sending with, and not explicitly require postfix. I think it's completely fine to suggest/require postfix if nothing is installed, because that makes it easy, but I've provided a `sendmail` command via another package, and I think my chef even removes postfix in favor of the msmtp package, and it's kinda frustrating :)

## Further Reading

I could link to msmtp documentation I guess: https://marlam.de/msmtp/