Change NavigationServer2D avoidance callbacks from Vector3 to Vector2

[smix8]

Changes NavigationServer2D avoidance callbacks from Vector3 to Vector2.

This was always an oddball close to a bug that no one wanted to change because it would be a (small) compatibility break.

It was the result of the NavigationServer2D using the NavigationServer3D behind the scene and no one adding a Vector2 callback to the api for 2d. It caused frequent annoyance for 2d users that tried to use it not expecting a Vector3 from a NavigationServer2D. E.g. raised in issues like #106250.

Now with the major change of a dedicated NavigationServer2D added in Godot 4.5 it is good time to change this as well.

NavigationAgent2D users are not affected as this is just the internals changing.

Projects that use the NavigationServer2D.agent_set_avoidance_callback() to create custom callbacks need to change their Vector3 parameter to Vector2 to not run into an error after this change.

Navigation documentation has likely script examples with this that need to be updated as well.

[AThousandShips]

Makes sense, the compatibility should be documented but I think it's worth breaking compatibility here, the callback should trigger an error if called with the wrong arguments unless I'm mistaken (worth testing) so it shouldn't be invisible

Don't think it's worth adding a whole new method to handle the new case and keeping the old one for compatibility

[akien-mga]

Thanks!