[gameplay-a24] Tweak elephant stable building time

As pointed out by Edwarf in the forum post, the elephant stable is rather expensive compared to the arsenal (both are city-phase structures).
This patch keeps the elephant stable's resource costs unchanged (it is visually larger than the other three military structures), but lowers the building time from 240 s to 180 s.

Differential revision: D3515
Reviewed by: @borg-
Patch by: @Nescio