Update the build system to use the .NET 6 SDK to build and to run the unit tests against .NET 6 and .NET Framework only.
As .NET 6 is an LTS release, Yarhl will stop supporting .NET Core 3.1 and .NET 5. However, as it targets .NET Standard 2.0, it should be compatible in those runtimes.
Description
Update the build system to use the .NET 6 SDK to build and to run the unit tests against .NET 6 and .NET Framework only. As .NET 6 is an LTS release, Yarhl will stop supporting .NET Core 3.1 and .NET 5. However, as it targets .NET Standard 2.0, it should be compatible in those runtimes.