use filter-dkimsign since it no longer crashes

stop documenting DKIMproxy because it's obsolete

fix filter name

add more angle brackets

add note for Debian 12

remove list

explain how to set cert permissions

don't specify the network interface

wrap variables in angle brackets

don't specify the network interface

there is no need to check where filters go

remove minimum version

OpenSMTPD 6.4 is old enough that there's no need mention it.

opensmtpd-filter-dkimsign is in Debian 12

rename filters

update to Debian 11

typo

switch to rspamd

Revert "switch to DKIMproxy"

This reverts commit 3ce791f8293b2a05d9f6f965ce052555c9e643be.

fix executables path

switch to DKIMproxy