Fix map format incompatibilities

This will fixes issues introduced by the content site adding RequiresMod to all maps and changing their IDs.
The map format has been bumped to 6, which requires RequiresMod to be defined. Any locally installed maps will be upgraded, keeping them consistent with the content site.

@ihptru will need to run `OpenRA.Utility.exe --upgrade-map <path>` in order for this to work.

The map editor will need testing to ensure that the PickAny option still works.