Update godot-cpp to the latest on the 4.1 branch

[dsnopek]

This updates the godot-cpp used by this extension to the latest on godot-cpp's 4.1 branch.

This will mean that the extension will only be compatible with Godot 4.1.4 or later due to the updates to the extension_api.json. However, if it was important than it still worked with Godot 4.1.0 to 4.1.3, then it should be possible to compile with scons custom_api_file=path/to/extension_api.json where that JSON file is from Godot 4.1.0 - and we could even stash that extension_api.json in the repo? Anyway, just let me know if you'd like adjustments for that.

However, updating to the newest godot-cpp that will work with Godot 4.1 will include a whole bunch of bug fixes that have been cherry-picked since then.

For example, PR godotengine/godot-cpp#1313 which fixes #239

As well as other various memory leaks, crashes, build-system improvements, etc.

[dsnopek]

Hm, not sure why SCons is failing on the CI, it's working for me locally...