fix(smtp-server): listen on all interfaces by default

This is the same behaviour as when using v1 configuration. Unlike the
web server which is proxied, most people are going to need this so having
the default remain seems like the easiest path for upgrades.

see #2852

lib/postal/config_schema.rb

@@ -233,7 +233,7 @@ module Postal
 
       string :default_bind_address do
         description "The default bind address the SMTP server should listen on unless overriden by the BIND_ADDRESS environment variable"
-        default "127.0.0.1"
+        default "::"
       end
 
       integer :default_health_server_port do