converts ex_act to legacy_ex_act, plants the seeds for new explosions (#4662)

Co-authored-by: VM_USER <VM_USER>

code/game/objects/structures/props/blackbox.dm

@@ -8,7 +8,7 @@
 	icon_state = "blackbox_off"
 
 // Black boxes are resistant to explosions.
-/obj/structure/prop/blackbox/ex_act(severity)
+/obj/structure/prop/blackbox/legacy_ex_act(severity)
 	..(++severity)
 
 

code/game/objects/structures/props/puzzledoor.dm

@@ -31,7 +31,7 @@
 		visible_message("<span class='cult'>\The [src] is completely unaffected by \the [Proj].</span>")
 	qdel(Proj) //No piercing. No.
 
-/obj/machinery/door/blast/puzzle/ex_act(severity)
+/obj/machinery/door/blast/puzzle/legacy_ex_act(severity)
 	visible_message("<span class='cult'>\The [src] is completely unaffected by the blast.</span>")
 	return