Put back -Wsign-conversion for gcc in CompilerWarnings.cmake #162

Issue #76 / PR #149 removed the -Wsign-conversion flag in CompilerWarnings.cmake. This may be OK for Clang and gcc using C, but it is not OK for gcc using C++.

The gcc documentation says:

In C, this option is enabled also by -Wconversion.

So NOT for C++.

See this demo for an example: https://godbolt.org/z/8YMbWr3Wx

Using gcc 15.2, compiling the code with -O3 -std=c++23 -Werror -Wconversion will work, but using -O3 -std=c++23 -Werror -Wconversion -Wsign-conversion will not.