infra: Migrate to .NET 6 (#2829)

* infra: Migrate to .NET 6

* Rollback version naming change

* Workaround .NET 6 ZipArchive API issues

* ci: Switch to VS 2022 for AppVeyor

CI is now ready for .NET 6

* Suppress WebClient warning in DoUpdateWithMultipleThreads

* Attempt to workaround System.Drawing.Common changes on 6.0.0

* Change keyboard rendering from System.Drawing to ImageSharp

* Make the software keyboard renderer multithreaded

* Bump ImageSharp version to 1.0.4 to fix a bug in Image.Load

* Add fallback fonts to the keyboard renderer

* Fix warnings

* Address caian's comment

* Clean up linux workaround as it's uneeded now

* Update readme

Co-authored-by: Caian Benedicto <caianbene@gmail.com>