Update save_mic_sound_to_file.dart

the method createJavaScriptNode on AudioContext is deprecated. It didnt work in the latest versions. Use createScriptProcessor() instead!

AudioContext.createJavaScriptNode()
Creates a JavaScriptNode, used for direct audio processing via JavaScript. This method is obsolete, and has been replaced by AudioContext.createScriptProcessor().
- https://developer.mozilla.org/en-US/docs/Web/API/AudioContext