Upgrade SixLabors.ImageSharp to v3

[aixasz]

This upgrade also fixed the issue that I found in my personal project also have custom code that reference to ImageSharp's lib with newest version. This will break the upload media feature in Piranha manager.

Method not found: 'SixLabors.ImageSharp.Image SixLabors.ImageSharp.Image.Load(System.IO.Stream, SixLabors.ImageSharp.Formats.IImageFormat ByRef)'.

Steps to reproduce:

1. Go to examples project, then add latest version of SixLabors.ImageSharp lib to the project.
2. Run project and go to media in manager.
3. Upload some image, the above error message will show up.

[aixasz]

@dotnet-policy-service agree