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 isn't a secondary objective by itself, as you need to do this in order to complete the primary objective.  You could make this more of an informational point (like the GDI04 missions) by changing this to something like "Destroy the SAM sites to receive air support".