Adjust jthread to C++20

[kuzkry]

Bjarne mentioned jthread in his talk: https://youtu.be/u_ij0YNkFUs?t=3165
I believe gsl::joining_thread should reflect that.

[hsutter]

Editors' call: gsl::joining_thread will be superseded by C++20 std::jthread. For C++17, people should use gsl::joining_thread and we won't rename it now to avoid disturbing existing users since it's going away anyway and they will eventually need to change to std::jthread. If you have a C++20 compiler, use std::jthread directly.

[kuzkry]

Thanks @hsutter, I agree with that.
But then probably as soon as C++20 is released, examples in the guidelines should be updated to use std::jthread instead of gsl::joining_thread, am I right? Similarly like with smart pointers, there are gsl counterparts of std::unique_ptr and std::shared_ptr but the guidelines prefer the ones from std.