Ported mission GDI03 to the new Lua API

Followup of https://github.com/OpenRA/OpenRA/pull/6786, https://github.com/OpenRA/OpenRA/pull/6769 and https://github.com/OpenRA/OpenRA/pull/6762.
This works in this case, since there are no heavy units besides the harvesters, but the correct fix would be to set the owner of the sbags to the Nod player instead of Neutral.