consider updating gsl::joining_thread to be std::jthread

[JordanMaples]

Microsoft::GSL has yet to implement joining_thread and with std::jthread being standardized, we should consider updating the documentation in the CoreGuidelines to std::jthread.

Today the Guidelines only mention C++17 features, this would imply adding documentation concerning C++20 features.

[JordanMaples]

microsoft/GSL#582

[kuzkry]

Hmm, maybe I wouldn't say std::, just jthread. C++20 isn't even out there and it will not be for a lot of people for a long time. Anyway, I've created a pull request (#1520) addressing this.

[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Editors' call: gsl::joining_thread will be superseded by C++20 std::jthread.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.

[hsutter]

Editors' call: Add a note to joining_thread that points to C++20 std::jthread.

[amitdo]

Please remove any mention of joining_thread from the C++ Core Guidelines.