"defence" vs "defense"
Freeciv codebase uses both spellings "defence" and "defense". Unify that to "defense" (US), as en_GB is a translation anyway.
There was no changes to translatable strings. Ruleset documentation gets touched,
Freeciv codebase uses both spellings "defence" and "defense". Unify that to "defense" (US), as en_GB is a translation anyway.