IMAP4rev2 / SMTPUTF8 Support (RFC 6531 & RFC 9051)

Is your feature request related to a problem? Please describe.

IMAP4rev2 is the latest revision of the IMAP protocol and brings a number of improvements, clarifications and standardisations compared to IMAP4rev1.

Why IMAP4rev2 is important:

1. IMAP4rev2 combines numerous extensions that were previously optional (e.g. ENABLE, UTF8=ACCEPT, LITERAL+, IDLE, etc.). .
2. Many older inconsistencies have been eliminated, simplifying implementations.
3. Modern mail servers (e.g. Dovecot, Cyrus) are already beginning to officially support IMAP4rev2.

Full support would make NextCloud future-proof and standards-compliant.

Proposal:

1. Gradual implementation of the mandatory extensions
2. Possibly a feature flag imap4rev2 for gradual activation

References:
RFC 9051 – IMAP4rev2
RFC 3501 – IMAP4rev1
RFC 6531 – SMTPUTF8

Describe the solution you'd like

Enable the creation of email accounts with non-ASCII characters (e.g. äöüß@domain.tld)

Describe alternatives you've considered

No response

Additional context

No response

Start by reviewing RFC 9051 for IMAP4rev2 and RFC 6531 for SMTPUTF8, then map the mandatory extensions and non-ASCII account creation requirement onto the existing mail implementation. Done means the requested protocol support and creation of addresses such as äöüß@domain.tld work reliably, but the issue does not identify files or tests to begin with.