Update C++ target doc to say that c++17 is now required

[tomqwpl]

C++ target documentation says C++11 is required, where in fact C++17 is required.

Specifically:

The C++ target supports all platforms that can either run MS Visual Studio 2013 (or newer), XCode 7 (or newer) or CMake (C++11 required).

Don't know when C++17 became a requirement, but there are usages of std::string_view which was added in C++17