Try the Wayland GDK backend, the engine now supports it (#66519)

* Try the Wayland GDK backend, the engine now supports it

* Update comment from review

* Just do default GTK behaviour

packages/flutter_tools/templates/app/linux.tmpl/main.cc

 #include "my_application.h"
 
 int main(int argc, char** argv) {
-  // Only X11 is currently supported.
-  // Wayland support is being developed: https://github.com/flutter/flutter/issues/57932.
-  gdk_set_allowed_backends("x11");
-
   g_autoptr(MyApplication) app = my_application_new();
   return g_application_run(G_APPLICATION(app), argc, argv);
 }