Release 4.0

[mrbean-bremen]

This is a housekeeping issue to discuss the contents of release 4.0, especially the supported frameworks, and the handling of the unsupported System.Drawing.Common (see also #929 and #910).
The release will have breaking changes, so this issue is also here to collect and discuss these and alert future users.

Planned changes:
[ ] Factor out drawing code and allow to use different libraries/frameworks for drawing (details to be discussed)
[ ] Support .NET 6
...

[H1Gdev]

By discontinuing support for older frameworks, we should be able to use the TylerBrinks/ExCSS. (There is a known issue and I am creating a PR.

[KoalaBear84]

Great to see. I'll check if #917 will be fixed, as I see it's also an issue in Svg.Skia library which I tried.

[AraHaan]

I can do the .NET 6 support, but it will be under net6.0-windows for now (Windows only).

[AraHaan]

Alright my .NET 6 change I feel should be ready to merge (for the Windows only branch).

As for other OS's I think it is best to look into alternatives to GDI+ for the rendering part.