Use complete Godot version for preprocessing conditionals

[Naros]

This expands on the current conditional logic for Godot versions by using the full Godot version. I suspect that when Godot's next major version happens, godot-cpp will need to continue to maintain compatibility with at least some Godot 4.x versions. This makes that possible.

[dsnopek]

I don't think this change makes these #ifs any more correct.

When Godot 5 happens, my plan was to reset the GDExtension interface such that all the old numbered functions are removed, and the latest becomes unnumbered. So, for example, classdb_register_extension_class5 (or whatever the latest is) will become just classdb_register_extension_class, and so the check for #if GODOT_VERSION >= 0x040500 to use the 5 version, will be incorrect.

If we want to account for Godot 5, I think it would be better to add something like this to a central location:

#if GODOT_MAJOR_VERSION != 4 || GODOT_MINOR_VERSION < 1
#error "This version of godot-cpp is only compatible with Godot 4.1 or newer"
#endif

That way when Godot 5 comes out, if folks try to use it with an old godot-cpp, they will get a nice message rather than cryptic errors about functions not existing. (Or, I guess we could add this check in the Python code before we even get to compiling C++?)

Of course, we could add #ifdefs to allow the same godot-cpp to work with Godot 4 and 5 if we wanted, but that's something we'll only be able to do correctly once we know exactly what Godot 5's GDExtension interface looks like