Failing HttpClientHttp2InteropTests on Windows 8.1

[JunTaoLuo]

Build: https://dev.azure.com/dnceng/public/_build/results?buildId=1214891&view=results

I noticed that a couple of the Http2 client interop tests started failing on Windows 8.1 with the following message:
System.NotSupportedException : HTTP/2 over TLS is not supported on Windows versions earlier than Windows 10 and Windows Server 2016 due to incompatible ciphers or missing ALPN support.

This seems pretty consistent. I'm not sure what changed but maybe the machines were updated and something messed up? Needs investigation but I'll disable the tests for now. Revert #34015 when we figure out why.

[Tratcher]

FYI these tests are new. I didn't notice on the PR because we only run Win8.1 nightly. Your fix is correct (skip on < Win10), no additional follow-up is needed.