Backporting physics fixes to 3.x

[elvisish]

This was closed as apparently not planned: #6158 (comment)

So I wanted to open discussion as to how feasible it would be to keep GodotPhysics on pairity with 3.x. 4.0 is a ways from being truly stable (regardless of the version naming) and many projects were started long before 4.0 was in alpha. Bullet physics are imprecise and Godot Physics in 3.x are very bugged, so I'm hoping it will be possible to have any fixes rolled back into 3.x for the final 3.6 release, ultimately allowing folks to finish their projects without having to upgrade to 4.0 but not having to deal with pre-existing bugs in the physics engine(s).

[elvisish]

Re-opened as a bug issue instead of a proposal: godotengine/godot#71961