Common template name change: hellenic_royal_stoa → stoa

The old name was unnecessary long, but also potentially misleading: stoas are Greek, but not necessary royal, and there are no other stoas in game. Just “stoa” is clearer, more correct, and more consistent with other template_structure_*.xml names.

Patch By: Nescio

Reviewed By: wraitii

Differential Revision: https://code.wildfiregames.com/D2036