ImageFormat.Webp always reverts to the png codec.

### .NET version

8.0, or any other version.

### Did it work in .NET Framework?

No

### Did it work in any of the earlier releases of .NET Core or .NET 5+?

_No response_

### Issue description

.NET 8.0 added ImageFormat.Webp but there is no such codec in GDI+. It always reverts to using the png codec.

### Steps to reproduce

bitmap1.Save("test.webp", ImageForma.Webp);
//or
ImageCodecInfo.GetImageEncoders(); // no webp encoder exists.