Require newer gnu g++ compiler

[bjorgve]

The code:

       file: src/vampyr/trees/trees.h
       .def(
            "saveTree",
            [](FunctionTree<D> &obj, const std::string &filename) {
                namespace fs = std::filesystem;
                obj.saveTree(filename);
                return fs::absolute(fs::path(filename + ".tree"));
            },
            "filename"_a)
        .def("loadTree", &FunctionTree<D>::loadTree, "filename"_a)

Appear to crash when g++ (Debian 8.3.0-6) 8.3.0 is used as compiler. But it work when I use the newer g++ (conda-forge gcc 12.2.0-19) 12.2.0

I'm not sure how to set the compiler requirement.