sympl-mail: sympl-mail-poppassd fails to start in Bullseye IPv6-only

It seems that on an IPv6-only instance running Bullseye falls fowl of a change in Ruby which prevents it from binding to 127.0.0.1, but adding a IPv4 address on loopback means it's okay, and this is fine with prior debian versions.

As a short-term work-around, adjusting https://gitlab.mythic-beasts.com/sympl/sympl/-/blob/bullseye/mail/sbin/sympl-mail-poppassd and changing:

EventMachine.run do
  begin
    EventMachine.start_server "127.0.0.1", port, Symbiosis::Email::PoppassHandler
  rescue StandardError => err
    syslog.info "Caught #{err.to_s} "
    EM.stop
  end
end

to:

EventMachine.run do
  begin
    EventMachine.start_server "127.0.0.1", port, Symbiosis::Email::PoppassHandler
  rescue StandardError => err
    begin
      EventMachine.start_server "::", port, Symbiosis::Email::PoppassHandler
    rescue StandardError => err
      syslog.info "Caught #{err.to_s} "
      EM.stop
    end
  end
end

Will have it fallback and still bind to 127.0.0.1. This also binds to other addresses, but it's firewalled so shouldn't be an issue.