Orca nerf: reducing damage vs. heavy 20 dmg -> 15

Orca rockets will do 15 damage vs. heavy. Hopefully this will take some edge off orca spam. This will:
-Give anti-air APC more lasting power vs. orca, ensuring it can at least down some of them and get their money's worth.
-Mammoth tank will bet better able to survive a mass assault and return fire with its rockets to down some of them (and get better money's worth).
-Give advanced guard towers better ability to defend themselves against air assault. (ATWR has heavy armor and does good damage, but can die quickly against a large group of orcas).
-Give armored units in general more resilience vs. orcas so they aren't completely helpless and quickly annihilated. Maybe then they can reach their target better (e.g. a couple tanks charging an artillery position that is being defended by some orcas).

Damage vs. infantry also slightly reduced as as result (also 20 -> 15 dmg).
Orcas generally aren't a threat to infantry, but E3s should be able to do some credible damage, even if they are in a small group. Enough orca spam will kill a small group of infantry, unfortunately. Since Orca has no business attacking infantry anyway, this slight nerf won't hurt orcas.

(Note: The damage model was simplified a bit, so that it does 100% of 30 dmg instead of 75% of 40 damage, vs. light & wood.)