ctucx.git: nixfiles

flake.nix: Update `simple-nixos-mailserver` to nixos-23.11

diff --git a/flake.nix b/flake.nix
@@ -171,7 +171,7 @@
       type  = "gitlab";
       owner = "simple-nixos-mailserver";
       repo  = "nixos-mailserver";
-      ref   = "nixos-23.05";
+      ref   = "nixos-23.11";
       inputs.nixpkgs.follows = "nixpkgs";
     };