[feature/chore] Switch away from net/smtp package for more configurability / flexibility of GtS email sending

[tsmethurst]

Right now we're using the standard library net/smtp package for sending emails, which is about as basic as you can get. This works fine mostly but it's finnicky and doesn't support more modern stuff like smtps.

We should look at switching to something else with a bit more functionality so we can provide more options for email configuration. Not sure if there's a "go-to" email package for this. Suggestions welcome.

Gathered here are smtp-related issues we should try to solve by making a switch:

• [feature] Support SMTPS (smtp with implicit tls) rather than just STARTTLS #3372
• [feature] Option to disable SMTP TLS cert validation #2973
• [bug/smtp] 504 Unrecognized authentication type when sending emails #3812

[rusangl]

Maybe it is possible to leave this job for mailutils app or something like that?