postfix: resolve main/master option deprecation

mail-server/postfix.nix

@@ -289,7 +289,7 @@ in
         forwards
       ]);
 
-      config = {
+      settings.main = {
         myhostname = cfg.sendingFqdn;
         mydestination = ""; # disable local mail delivery
         recipient_delimiter = cfg.recipientDelimiter;
@@ -429,7 +429,7 @@ in
       submissionOptions = submissionOptions;
       submissionsOptions = submissionOptions;
 
-      masterConfig = {
+      settings.master = {
         "lmtp" = {
           # Add headers when delivering, see http://www.postfix.org/smtp.8.html
           # D => Delivered-To, O => X-Original-To, R => Return-Path