Update SmtpUTF8Mailbox support to RFC 9598

[davidben]

RFC 9598 was just published. It retracts RFC 8398 so that the domain names are expected to use A-labels. See this section for details:
https://www.rfc-editor.org/rfc/rfc9598.html#section-8

This means verifiers are no longer expected to evaluate punycode to convert name constraints. This change was motivated by OpenSSL's CVE-2022-3602 and CVE-2022-3786, because X.509 validators should not need punycode implementations in the first place. See also https://mailarchive.ietf.org/arch/msg/spasm/0D_ljgmD8y6J2OZ9L6GQubLSO9Q/

If you all update to the new one, you should be able to delete the punycode implementation.