Migrate to .NET 6

[Inerska]

Hey new contributor here.

.NET 6 is the replacement of .NET 5 and need to be migrated, it's not supported anymore.

Thanks, hope this repo is still active or I couldn't contribute :/

[justinmk]

This repo needs someone to take ownership of it. Migrating the CI to GHA would be a good first step, which might also then be fast-followed by a migration to .NET 6.

[kaashyapan]

I see the last commit #14 has migrated the project from Appveyor to GH CI on Sep 9, 2021.
Can the project now be migrated to .NET 6 ?