Latest main does not build on macOS due to some missing C++17 features in Apple Clang

While testing the latest main on macOS, I got an error due to a missing C++17 feature in Apple Clang.

We saw others of those in the past; for example, Apple Clang did not support std::filesystem until recently.

This time is the turn of std::from_chars:

/Users/rbianchi/work/atlas/geomodel/test_main_Jan24/GeoModel/GeoModelTools/GeoModelFuncSnippets/src/StringUtils.cxx:123:13: error: call to deleted function 'from_chars'
        if (std::from_chars(str.data(), str.data() + str.size(), number).ec !=  std::errc{}) {
            ^~~~~~~~~~~~~~~

For which, I found a comment here: https://stackoverflow.com/a/74962482

I would ping @johannes, so he knows.

Cc: @boudreau @tsulaia