Add .NET Standard 2.0 / .NET 6 targets

[RichardD2]

Fixes #7

[RichardD2]

Looks like the EncodeAndDecode_LargeRange_ReturnsUniqueIdsAndRoundTripsSuccessfully test is timing out on the 100_000_000 case now that there are three target frameworks to test.

[aradalvand]

Looks like the EncodeAndDecode_LargeRange_ReturnsUniqueIdsAndRoundTripsSuccessfully test is timing out on the 100_000_000 case now that there are three target frameworks to test.

@RichardD2 Are you sure that's the problem? I'm seeing a System.TypeInitializationException in the action logs here on the net472 test. Not sure what it means.

[RichardD2]

That's odd; the tests run fine for me in Visual Studio, albeit slowly on that specific test.

Perhaps it's because the tests are running on Ubuntu, which doesn't support .NET Framework?

tests.yaml
runs-on: ubuntu-latest

And they're only configured to use .NET 7:

dotnet-version: "7.x" # TODO: Specify multiple .NET versions once we add support for other target frameworks

[aradalvand]

@RichardD2 Thank you!