Use NixOS 21.11

configuration.nix

@@ -3,7 +3,7 @@ let
   secrets = import ./secrets.nix;
   hostname = "mail-dumpstack-io";
   domain = "dumpstack.io";
-  branch = "21.05";
+  branch = "21.11";
 in {
   imports = [
     ./hardware-configuration.nix