Make default SocketsHttpHandler SslProtocols include Tls11/12 on Win7/2008R2 (dotnet...

Make default SocketsHttpHandler SslProtocols include Tls11/12 on Win7/2008R2 (dotnet/corefx#28788)

Windows 7 and Windows 2008 R2 support TLS 1.1 and 1.2, but for legacy reasons by default those protocols are not enabled when a developer elects to use the system default.  However, in .NET Core 2.0 and earlier, HttpClientHandler would enable them, due to being a wrapper for WinHTTP, which enabled them.  Both for compatibility and because we prefer those higher protocols whenever possible, SocketsHttpHandler also pretends they're part of the default when running on Win7/2008R2.

Commit migrated from https://github.com/dotnet/corefx/commit/0c7a1de0448c9dee76723d0bedd90f76c3436662