<ciso646> is removed in C++20 and should not be included

[matbech]

VS 17.3 Preview 4 fails to compile PEGTL with C++20 or higher:
Error:

C:\Program Files\Microsoft Visual Studio\2022\Preview\VC\Tools\MSVC\14.33.31627\include\ciso646(15,22): error C4996: '_Header_ciso646': warning STL4036: <ciso646> is removed in C++20. You can define _SILENCE_CXX20_CISO646_REMOVED_WARNING or _SILENCE_ALL_CXX20_DEPRECATION_WARNINGS to acknowledge that you have received this warning.

ciso646 should not be included in demangle.hpp:
https://github.com/taocpp/PEGTL/blob/main/include/tao/pegtl/demangle.hpp#L8
The <version> header should be used instead on C++20

[ColinH]

Looking at demangle.hpp I'd say this header is probably not needed (anymore), I'll remove it and see whether all CI jobs agree.