Update Piranha.ImageSharp dependency SixLabors.ImageSharp to v3

[jmbryan4]

update to ImageSharp 3.x see: https://sixlabors.com/posts/announcing-imagesharp-300/

[tidyui]

There are some major changes to the licensing between v2 and v3 of ImageSharp that we need to clear out before being able to update.

[usercode]

Updating the ImageSharp dependency is also important because of breaking changes between version 2 and 3.

I use Piranha CMS with other open source library which target the version 3.x.

If I try to upload an image with the manager, I get this error message.

I think there is no license issue. (It's allowed to use ImageSharp as "Transitive Package Dependency" by open source projects like Piranha CMS.)

https://github.com/SixLabors/ImageSharp/blob/main/LICENSE

[tidyui]

@usercode We can't upgrade ImageSharp to a new major version with breaking changes without changing major version for Piranha itself. If you need to upgrade to the latest version of ImageSharp in your project before this it's very easy to create your own local pre-release version of Piranha.ImageSharp and reference you local package in your project.