Follow MailPace rebranding

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](https://docs.npmjs.com/policies/unpublish)
- [ ] 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.