postfix: ETRN discarded from EHLO silently

After the 2.14.4 rollout the test server's mail.log held "discarding EHLO
keywords: ETRN" for every connection on every port: smtpd logs a discard
unless the mask has EHLO_MASK_SILENT (Postfix 3.10.13, src/smtpd/smtpd.c:2016;
src/global/ehlo_mask.c:88 "SILENT-DISCARD"; postconf(5)
smtpd_discard_ehlo_keywords, "Specify the silent-discard pseudo keyword to
prevent this action from being logged"). smtpd_discard_ehlo_keywords is now
silent-discard,etrn; ETRN is still refused (smtpd_etrn_restrictions=reject)
and still not offered.

make e2e (64 passed; checkPostfixHardening reads the value back,
testInboundRelay still sees no ETRN in EHLO and 554) and go vet + go test
(26 packages) on the test server for this tree.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/architecture.md

@@ -104,7 +104,7 @@ smtp/inet` removes Debian's stock smtpd). Outbound delivery still uses the
   submission chain would otherwise let a signing outage defer relayed mail and
   would file inbound mail into the outbound send log under the sender's
   domain. `ETRN` is refused on every listener and not offered in the EHLO
-  reply (`smtpd_etrn_restrictions=reject`, `smtpd_discard_ehlo_keywords=etrn`):
+  reply (`smtpd_etrn_restrictions=reject`, `smtpd_discard_ehlo_keywords=silent-discard,etrn`):
   Postfix's default lets any client flush the queue toward an inbound
   domain's upstream. With the inbound relay on the chain is the optional antispam milter,
   then the panel's **inbound journal milter** (unix socket