games/freeciv21: Update 3.1.1 => 3.1.2

While here, drop the custom variable FC21_VERSION and use standard
variables instead.

Changelog: https://github.com/longturn/freeciv21/releases/tag/v3.1.2

PR: 297212
Reported by: Tobias Rehbein <tobias.rehbein@web.de> (maintainer)