Pre-burns the mineral planet (#6590)

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## About The Pull Request

A planet can't have an atmosphere filled with fuel and oxidizer at the
same time, except trace amounts of one or the other. If a planet's
atmosphere is sufficient for fire to spread, then a single spark of any
sort anywhere (such as, say, lightning, or a rock rolling down a hill
just right) will cause the whole planet to catch fire.

This changes the Class G mineral planet so that it's already done that.

## Why It's Good For The Game

I mean having a burn mix planet is genuinely a little funny but it's:

1. Not actually that fun gameplay-wise (it just means nobody ever goes
there)
2. Completely goofy setting-wise (for reasons mentioned)
3. Phoron's awful on its own, it doesn't need to catch fire

## Changelog

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:cl:
tweak: Oxygen on the Class G Mineral-Rich planet changed to CO2
/:cl:

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code/modules/maps/weather/classg.dm

@@ -1,6 +1,6 @@
 /datum/atmosphere/planet/classg
 	base = list(
-		/datum/gas/oxygen = 10,
+		/datum/gas/carbon_dioxide = 10,
 		/datum/gas/nitrogen = 10,
 		/datum/gas/phoron = 80,
 	)