Update instancing.zip for Godot 4

[florlbright]

This commit slightly changes Main.gd in the instancing sample, so it works with 4.0.

Used the migrate project thing, only had to change 1 thing manually: export (PackedScene) var Ball changed to var Ball = load("res://Ball.tscn")

Closes https://github.com/godotengine/godot-docs/issues/6564.

[Calinou]

We should really move this demo to the godot-demo-projects repository instead of updating a ZIP file. Otherwise, we'll keep bloating the Git history with a binary file every time the demo has to be updated for any reason.

[andreymal]

Both old and new instancing.zip have smaller collision shapes when importing to Godot 4. Is it some migration bug?

[skyace65]

Superseded by #7222