Set MediaWiki 1.31 as minimum requirement in extension.json

Added an old, but not to-outdated required version to keep a minimum
requirement

Change-Id: I4297eea83fd83b4ddf7883bec1364ce2b5f9ec86

extension.json

@@ -10,6 +10,9 @@
 	"url": "https://www.mediawiki.org/wiki/Extension:RandomSelection",
 	"descriptionmsg": "randomselection-desc",
 	"type": "parserhook",
+	"requires": {
+		"MediaWiki": ">= 1.31.0"
+	},
 	"MessagesDirs": {
 		"RandomSelection": [
 			"i18n"
@@ -25,5 +28,5 @@
 		"ParserFirstCallInit": "RandomSelection::register",
 		"MagicWordwgVariableIDs": "RandomSelection::variableIds"
 	},
-	"manifest_version": 1
+	"manifest_version": 2
 }