Remove clanModules

- Removes clanModules in favor of the new clanServices
- Adds a warning and link to the migration guide

pkgs/installer/flake-module.nix

@@ -12,7 +12,6 @@ let
       imports = [
         ./iwd.nix
         self.nixosModules.installer
-        self.clanModules.trusted-nix-caches
       ];
 
       system.stateVersion = config.system.nixos.release;