c++: Some C++20 and C++23 option help fixes

c++: Some C++20 and C++23 option help fixes

I've noticed we still refer to C++20 as draft standard, and there is a pasto
in C++23 description.

2021-01-28  Jakub Jelinek  <jakub@redhat.com>

	* c.opt (-std=c++2a, -std=c++20, -std=gnu++2a, -std=gnu++20): Remove
	draft from description.
	(-std=c++2b): Fix a pasto, 2020 -> 2023.