Switch out mail library (again)

[Jumpy-Squirrel]

https://github.com/Shopify/gomail has been archived and is no longer being maintained.

We should see about switching to another library or implementation.

Possible candidates:

• https://github.com/xhit/go-simple-mail from @xhit
• https://github.com/wneessen/go-mail from @wneessen

Thanks to @Neustradamus for the heads up.

[Neustradamus]

@Jumpy-Squirrel: Thanks for your answer and this new ticket!

• replace abandoned mail library #52

For information, Gitea has recently migrated to:

• https://github.com/wneessen/go-mail from @wneessen

[Neustradamus]

@Jumpy-Squirrel: Good job, thanks for your PR!

• migrate to wneessen/go-mail - Shopify/gomail is abandonware #119