Set mw version requirement to 1.31

I don't actually know if it's 1.31 that it requires, but it
definitely doesn't work in 1.27, the previous LTS. And we're on 1.32
anyway now.

Change-Id: I8bd8dcc8a58f605e79d4f0c92f58febcad834f6a

skin.json

@@ -7,6 +7,9 @@
 	"namemsg": "skinname-timeless",
 	"license-name": "GPL-2.0-or-later",
 	"type": "skin",
+	"requires": {
+		"MediaWiki": ">= 1.31.0"
+	},
 	"ValidSkinNames": {
 		"timeless": "Timeless"
 	},