The idea for this patch emerged from the discussion in D2846.
Nisean war horses were reserved for the heaviest shock cavalry (i.e. champion spearmen in 0 A.D.) This patch makes the `special_war_horses.json` technology similar to the `spartans_agoge.json` technology:
* instead of all cavalry, limited to Champion Cavalry Spearman;
* because +20% health is a lot, the training time is raised by 10%;
* cost raised by 200 food to 400 food, 200 metal;
* made the new “Horse Breeding” technology (D3317/rP24548) a requirement.
(For comparison, `upgrade_mace_silvershields.json` costs 600 metal.)