doc: Remove redundant sentence about modules being in C++20

As C++20 has already been published, we don't need to link to the draft
(which is now the C++23 draft anyway). And there's no need to say it's
part of the C++20 spec, or that there might be defect reports. That's
true for everything in C++20, so calling it out here just for Modules
isn't needed.

gcc/ChangeLog:

	* doc/invoke.texi (C++ Modules): Remove anachronism.