sombrero: Use transmission v4

As v3 is being deprecated

hosts/sombrero/modules/transmission/default.nix

@@ -13,6 +13,7 @@ in
     services = {
       transmission = {
         enable = true;
+        package = pkgs.transmission_4;
         openFirewall = true;
         openRPCPort = true;
         settings.rpc-port = 9191;