Tweak GitHub Actions workflow to run tests on multiple TargetFrameworks in parallel

[rmunn]

Currently the Windows tests are run against net461 and net6.0, and #332 adds net8.0. Once #332 is merged it will take about 2 hours to run the Windows tests, which run in series rather than in parallel. It would be nice to use a matrix to specify the target framework to run the tests against, so that net461, net6.0, and net8.0 tests can all run in parallel on different runners. This would take the runtime of the CI build from 2 hours to roughly 40-45 minutes.