Simplify RenderWidgetHostViewCocoa Gesture Handling Logic

This change has the following potential impacts on WebView2Mac: 1) Interface change impact: Due to significant changes in the mojo interfaces of Remote Cocoa (GestureBegin/GestureUpdate/GestureEnd merged into PinchEvent, SmartMagnify renamed to SmartMagnifyEvent), if WebView2Mac uses these interfaces, the code needs to be updated accordingly to maintain compatibility; 2) Gesture handling logic change: The complex gesture state management and staged processing logic have been removed and replaced with unified PinchEvent handling, which may affect the handling behavior of zoom gestures in WebView2Mac; 3) Threshold check simplification: The simplification of zoom threshold checking logic may impact the mechanism in WebView2Mac that prevents accidental zooming; 4) Backward compatibility: The removal of compatibility code for earlier macOS versions means that if WebView2Mac needs to support older macOS versions, it may need to handle related compatibility issues independently. Overall, these changes mainly affect gesture input handling, especially zoom-related functionality.