Bumped the minimum C++ version from 11 to 17 in docs

[Guiorgy]

In the getting started guide for Linux:

C++ compiler with at least C++1117 support.

In the getting started guide for MacOS:

If GCC throws errors and your program does not compile, you may be using C++03 instead of ≥C++1117. Use the flag -std=c++~11~17.