Add support for .NET 6

[pellared]

https://www.nuget.org/packages/OpenTracing/0.12.1 does not support .NET 6 as it dropped support for .NET Standard 2.0.

Can we additionally target netstandard2.1?

References:

• https://devblogs.microsoft.com/dotnet/the-future-of-net-standard/
• https://www.infoworld.com/article/3608611/whats-new-in-microsoft-net-6.html

[pellared]

Closing. I was wrong.

Support for older frameworks is being dropped, including frameworks older than .NET Framework 4.6.1, .NET Core 3.1, and .NET Standard 2.0.