nixos/postfix: fix compatibility level

Postfix has started outputting an error on startup that it can't parse
the compatibility level 9999.

Instead, just set the compatibility level to be identical to the current
version, which seems to be the (new) intent for the compatibility level.

nixos/modules/services/mail/postfix.nix

@@ -773,7 +773,7 @@ in
         };
 
       services.postfix.config = (mapAttrs (_: v: mkDefault v) {
-        compatibility_level  = "9999";
+        compatibility_level  = pkgs.postfix.version;
         mail_owner           = cfg.user;
         default_privs        = "nobody";