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 can now be done by granting an upgrade instead of spawning an actor, but that's a minor detail that could probably be left for a future pr.