Fix map format incompatibilities

This per-emptively fixes issues with the recent content site changing all the map IDs.
The map format has been bumped to 6, which requires RequiresMod to be defined.

@ihptru will need to run `OpenRA.Utility.exe --upgrade-map <path>` on the hosted maps to make everything consistent.

The map editor will need testing to ensure that the PickAny option still works.