Rename metal sheets to iron sheets (#56643)

It's a specific type of metal, it shouldnt just be called generic "metal".
The reagent, ore and material datum are already called iron.

code/modules/mining/equipment/survival_pod.dm

@@ -227,7 +227,7 @@
 	desc = "A large machine releasing a constant gust of air."
 	anchored = TRUE
 	density = TRUE
-	var/buildstacktype = /obj/item/stack/sheet/metal
+	var/buildstacktype = /obj/item/stack/sheet/iron
 	var/buildstackamount = 5
 	CanAtmosPass = ATMOS_PASS_NO
 

code/modules/mining/ores_coins.dm

@@ -87,7 +87,7 @@
 	singular_name = "iron ore chunk"
 	points = 1
 	mats_per_unit = list(/datum/material/iron=MINERAL_MATERIAL_AMOUNT)
-	refined_type = /obj/item/stack/sheet/metal
+	refined_type = /obj/item/stack/sheet/iron
 	mine_experience = 1
 	scan_state = "rock_Iron"
 	spreadChance = 20