Not THAT Slow - Update Movespeed Modifiers for Smooth Movement (#22127)

The /tg/-style movespeed_modifier datums were not incorporated into the
original smooth movement PR. While
https://github.com/Aurorastation/Aurora.3/pull/22043 restores that
functionality, the original values no longer feel good now that we're on
smooth movement. This PR halves ALL movespeed_modifier/slowdown datums,
with the exception of species-native, as a baseline before more
individual adjustments are made.

code/modules/overmap/ship_weaponry/_ship_ammunition.dm

@@ -4,7 +4,7 @@
 	icon = 'icons/obj/projectiles.dmi'
 	icon_state = "nuke"
 	w_class = WEIGHT_CLASS_HUGE
-	slowdown = 2
+	slowdown = 1
 	drop_sound = 'sound/items/drop/shell_drop.ogg'
 	var/projectile_type_override //Override projectile type fired by the gun. This is because certain guns don't use ammo (the Leviathan) but with some we want the ammo to matter.
 	var/overmap_projectile_type_override //Override projectile type on the overmap, fired by the gun. Like the Grauwolf Probe.

code/modules/overmap/ship_weaponry/weaponry/longbow_ammo.dm

@@ -126,7 +126,7 @@
 	warhead_state = "high_ex"
 	caliber = SHIP_CALIBER_406MM
 	warhead_type = SHIP_AMMO_IMPACT_HE
-	slowdown = 2
+	slowdown = 1
 	var/drop_counter = 0
 	var/cookoff_devastation = 0
 	var/cookoff_heavy = 3