upgrade nixpkgs to NixOS 23.11

sp-modules/simple-nixos-mailserver/config.nix

@@ -1,4 +1,4 @@
-mailserverDate: { config, lib, ... }:
+{ config, lib, ... }:
 let
   sp = config.selfprivacy;
 in
@@ -66,11 +66,7 @@ lib.mkIf sp.modules.simple-nixos-mailserver.enable
       "admin@${sp.domain}" = "${sp.username}@${sp.domain}";
     };
 
-    certificateScheme =
-      if builtins.compareVersions mailserverDate "20230525011002"
-        >= 0
-      then "manual"
-      else 1;
+    certificateScheme = "manual";
     certificateFile = "/var/lib/acme/${sp.domain}/fullchain.pem";
     keyFile = "/var/lib/acme/${sp.domain}/key.pem";