Fixes cas targeting incorrect signals (#6459)

# About the pull request
Closes #6447, untested. 

# Explain why it's good for the game
bug bad

# Changelog
:cl:
fix: fixes cas being able to target shipside signals
/:cl:

---------

Co-authored-by: DOOM <N/A>

code/game/machinery/computer/dropship_weapons.dm

@@ -621,7 +621,8 @@
 	. = list()
 	var/datum/cas_iff_group/cas_group = GLOB.cas_groups[faction]
 	for(var/datum/cas_signal/LT as anything in cas_group.cas_signals)
-		if(!istype(LT) || !LT.valid_signal())
+		var/obj/object = LT.signal_loc
+		if(!istype(LT) || !LT.valid_signal() || !is_ground_level(object.z))
 			continue
 		var/area/laser_area = get_area(LT.signal_loc)
 		. += list(