Update Makefile to run net461 only if the windows os is detected

[malandis]

Currently our Makefile has different targets for testing against .NET 6.0 + .NET Framework 4.61 vs .NET 6.0 only vs .NET Framework 4.61. The latter tests, .NET Framework 4.61, only work on Windows. We can collapse makefile targets into a single target that only tests against .NET Framework 4.61. if Windows is detected, else .NET 6.0 only.

Also consider updating the test runner to exclude .NET Framework 4.61 tests based on the host OS.