[4.4] Need update `create_instance_func` because godot gdextension_interface.h upgrade

[pupil1337]

Godot version

godot master branch

godot-cpp version

godot-cpp master branch

System information

windows 10, msvc, i7-9700

Issue description

Use deprecated=no build godot will has this problem, because gdextension_classdb_construct_object be marked deprecated. Please update the new function. (I didn't submit a PR because this upgrade maybe need modify memnew's postinitialize i can`t handle it.)

Steps to reproduce

1. Build godot custom args use deprecated=no. (godot: scons target=editor dev_build=yes... deprecated=no)
2. compile godot-cpp will successed.
3. open godot editor will error because cant find gdextension_classdb_construct_object.

Minimal reproduction project

N/A