3D kinematic character jumping/jittering on flat surface

https://github.com/user-attachments/assets/789cee49-37ab-44fd-a56c-9d8ad73e0273

## Steps to reproduce

1. Clone this repo
2. Multiply artificial gravity of the [`character_movement_from_inputs` function](https://github.com/dimforge/rapier/blob/2f9d9ba94b33a188eff70b72ac154bbba1292868/examples3d/utils/character.rs#L89) by x100.
(The effect is still present with regular gravity but becomes more noticeable when it is cranked up)

```rs
if artificial_gravity {
desired_movement -= Vector::y() * speed * 100.0;
}
```

3. Run the `examples3d` demo by following the README instructions
4. Observe that the character jumps / jitters around when moving on the flat ground surface

## Expected result

It would be preferable if the character moved smoothly despite having high kinematic gravity