VirtualThread.notifyJvmtiDisableSuspend should be static

This method disables/enables suspend of the current virtual thread, a no-op if the current thread is a platform thread. It is confusing for this to be an instance method, it should be static to make it clearer that it doesn't change the target thread.
The method VirtualThread.notifyJvmtiHideFrames also should be static as it does not use/need the virtual thread `this` argument.