Switch to newer C++ standard

[wtraylor]

At some point, the codebase should be updated to C++17 or C++20. The reason why I didn’t use these standards in the first place was because even C++17 still seemed too new at the time to be available on typical workstations.

Here I list what should be considered when switching:

Use deprecated attribute (C++14):

• Fauna::World::simulate_day(const Fauna::Date& date, const bool)
• Fauna::World::World()

Use std::filesystem (C++17) for functions in src/Fauna/filesystem.h.