Update KDE module to Plasma 6

Flake lock file updates:

• Updated input 'disko':
    'github:nix-community/disko/8ea5bcccc03111bdedaeaae9380dfab61e9deb33' (2024-05-30)
  → 'github:nix-community/disko/9d5c673a6611b7bf448dbfb0843c75b9cce9cf1f' (2024-05-30)
• Updated input 'nixpkgs':
    'github:NixOS/nixpkgs/9d29cd266cebf80234c98dd0b87256b6be0af44e' (2024-05-25)
  → 'github:NixOS/nixpkgs/25cf937a30bf0801447f6bf544fc7486c6309234' (2024-05-29)
• Updated input 'nixpkgs-unstable':
    'github:NixOS/nixpkgs/9ca3f649614213b2aaf5f1e16ec06952fe4c2632' (2024-05-27)
  → 'github:NixOS/nixpkgs/ad57eef4ef0659193044870c731987a6df5cf56b' (2024-05-29)

modules/desktop/kde.nix

@@ -10,13 +10,15 @@ in {
   };
 
   config = lib.mkIf cfg.enable {
-    services.xserver = {
-      enable = true;
+    services = {
+      xserver = {
+        enable = true;
+      };
+      desktopManager.plasma6.enable = true;
       displayManager.sddm = {
         enable = true;
         wayland.enable = true;
       };
-      desktopManager.plasma5.enable = true;
     };
 
     networking.networkmanager.enable = true;