Change German translation of cannons and rail guns

* classify cannons, rail guns and the EMP cannon as "Kanone" instead of
  "Geschütz", a less specific term that includes mortars and howitzers
* in #326, all flak weapons had been re-translated as "Flugabwehrkanone"
  (replacing the term "Flugabwehrgeschütz") to match the acronym "FLAK"
  as part of 4b2a947

Refs #326
Fixes #328

Co-authored-by: highlander1599 <highlander1599@gmail.com>
Co-authored-by: kreuvf <kreuvf@users.noreply.github.com>