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
update to Debian 11
switch to rspamd
Revert "switch to DKIMproxy"

This reverts commit 3ce791f8293b2a05d9f6f965ce052555c9e643be.
fix executables path
switch to DKIMproxy