postfix ≥3.0: don't advertise SMTPUTF8 support.

We're relaying messages to our LMTP daemons (Dovecot, Amavisd) and some
downstream SMTP servers, not all of which are under our control.
Forwarding messages with UTF-8 envelope addresses or RFC 5322 headers
yields undeliverable messages, and the bounces make us a potential
backscatter source.  So it's better to disable SMTPUTF8 at this point.

Cf. also http://www.postfix.org/SMTPUTF8_README.html and
https://unix.stackexchange.com/questions/320091/configure-postfix-and-dovecot-lmtp-to-receive-mail-via-smtputf8 .

See also upstream's comment at https://marc.info/?l=postfix-users&m=149183235529042&w=2 :

    “Perhaps SMTPUTF8 autodetection could be more granular: UTF8 in the
     envelope is definitely problematic for a receiver that does not
     support SMTPUTF8, while UTF8 in a message header is less so.”

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