Another NERF to heavy revolver ammo (#6775)

# About the pull request
#4706
So this PR replaced knockdown with slow+knockback, but unlike the
knockdown proc the slow proc didn't check for the mob size, so you
became able to slow down T3s and even Queen with heavy revolver ammo,
which clearly wasn't intended. This wasn't noticed until recently.

# Explain why it's good for the game
Free slows with no telegraph on T3s is not something we need in the
game.

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

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screenshots and the `<details>` tags.

</details>


# Changelog
:cl: ihatethisengine2
balance: heavy revolver ammo cannot slowdown t3s anymore.
/:cl:

code/datums/ammo/ammo.dm

@@ -173,6 +173,10 @@
 		living_mob.apply_stamina_damage(fired_projectile.ammo.damage, fired_projectile.def_zone, ARMOR_BULLET)
 
 /datum/ammo/proc/slowdown(mob/living/living_mob, obj/projectile/fired_projectile)
+	if(isxeno(living_mob))
+		var/mob/living/carbon/xenomorph/xeno = living_mob
+		if(xeno.caste.tier > 2 || (xeno.caste.tier == 0 && xeno.mob_size >= MOB_SIZE_BIG))
+			return //tier 3 and big tier 0 (like queen) are not affected
 	if(iscarbonsizexeno(living_mob))
 		var/mob/living/carbon/xenomorph/target = living_mob
 		target.apply_effect(1, SUPERSLOW)