Add `[Obsolete]` to `ID2D1Factory::GetDesktopDpi()`

[Rick Brewster]

When you compile C/C++ code with #include <d2d1.h>, and try to call this method, you get a warning/error because the method is marked as deprecated:

/// <summary>
/// Retrieves the current desktop DPI. To refresh this, call ReloadSystemMetrics.
/// </summary>
[[deprecated("Deprecated. Use DisplayInformation::LogicalDpi for Windows Store Apps or GetDpiForWindow for desktop apps.")]]
STDMETHOD_(void, GetDesktopDpi)(
    _Out_ FLOAT *dpiX,
    _Out_ FLOAT *dpiY 
    ) PURE;

It would be nice if the corresponding TerraFX methods could be marked as [Obsolete].