Run tests on Windows and also against .NET FX

[stevejgordon]

Our unit and integration tests currently run against .NET 6 target framework and in CI, run on Linux runners. Occasionally there may be Windows or .NET FX bugs that we do not catch with our current testing. It would be nice to apply similar changes as proposed for Elastic.Transport which ensures that on Windows, we also run the tests against the net462 target. We can then introduce a Windows-based action to trigger those tests.