Buff to Smartgunner AP ammo (#7473)

# About the pull request

This PR is a follow-up to #7428 where AP ammo didn't get adjusted since
fall off values are flat not percentage.

Essentially now SG regular ammo will be at 0.667% power on screen edge
(20) and armor penetrating ammo will be at 0.7% power on screen edge
(14). Prior to this change, armor penetrating ammo would be at 50% power
on screen edge (10).

# Explain why it's good for the game

Was not intended for one ammo type to be nerfed heavier than the other
for SG.

# Testing Photographs and Procedure
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<summary>Screenshots & Videos</summary>

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screenshots and the `<details>` tags.

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# Changelog
:cl: Drathek
balance: Buffed SG armor penetration ammo damage falloff from 5 to 3.
/:cl:

code/datums/ammo/bullet/special_ammo.dm

@@ -20,6 +20,7 @@
 	name = "armor-piercing smartgun bullet"
 	icon_state = "bullet"
 
+	damage_falloff = DAMAGE_FALLOFF_TIER_8
 	accurate_range = 12
 	accuracy = HIT_ACCURACY_TIER_2
 	damage = 20