Merge branch '6428-postfix-advertises-smtputf8-but-local-lmtp-delivery-fails-because-dovecot-does-not-support-it' into 'develop'

Resolve "Postfix advertises SMTPUTF8 but local LMTP delivery fails because dovecot does not support it"

Closes #6428

See merge request ispconfig/ispconfig3!1727

Author: Till Brehm

install/tpl/postfix_3-0.conf.master

@@ -14,3 +14,6 @@
 # validate DANE
 smtp_dns_support_level = dnssec
 smtp_tls_security_level = dane
+
+# Disable SMTPUTF8 (until Dovecot supports it: https://git.ispconfig.org/ispconfig/ispconfig3/-/issues/6428)
+smtputf8_enable = no