Follow MailPace rebranding

[franky47]

OhMySMTP is now known as MailPace.
https://blog.mailpace.com/blog/ohmysmtp-is-now-mailpace/

Checklist for rebranding update:

• Publish a note on fastify-ohmysmtp indicating the name change & mark packages as deprecated
• Rename this repository to 47ng/fastify-mailpace
• Update code references
• Add migration documentation from v1/ohmysmtp to v2+/mailpace
• Publish to fastify-mailpace on NPM

Needless to say, this will be a breaking change, so it will cause a major version bump (v2).

Possible footguns to watch out for:

If you entirely unpublish all versions of a package, you may not publish any new versions of that package until 24 hours have passed.