#323 Add net 6.0

[thompson-tomo]

Adds in net 6.0 so that Microsoft.CSharp is no longer required as a dependency and improve cancellation propagation

Closes #323

[marcin-krystianc]

Thanks @thompson-tomo for your patience and apologies for the delay with this PR.

Initially, I was hesitant about this change because our test matrix in the CI wasn't ready for adding a new target framework. Also, adding a new target framework to the library expands the API surface of the library that we need to make sure is tested.

But we've decided now that adding new tfms is inevitable, so we are going to move forward with this change.

• Since .net6.0 is already out of support, should we update this PR to use net8.0 instead (current LTS)?

• Please add an older target framework (I guess the previous LTS, which is net6.0) to the test project so we are able to run tests for netstandard2.0.

• Please update the CI matrix so we run tests for net461, net.6.0, and net8.0.