games/wesnoth: Revert version 1.13 => 1.12

This port is supposed to track stable releases. Those end in even
numbers, e.g. 1.10, 1.12, 1.14. The odd numbers are development
releases. The upgrade to 1.13 was due to a misunderstanding about the
version numbering scheme.

PR: 200236
Submitted by: maintainer (Torsten Zuehlsdorff)