build: Updating mediawiki/mediawiki-codesniffer to 20.0.0

Change-Id: I32072728a090a56d9e2c4abac5497e6bb66b8d52

composer.json

@@ -1,7 +1,7 @@
 {
 	"require-dev": {
 		"jakub-onderka/php-parallel-lint": "1.0.0",
-		"mediawiki/mediawiki-codesniffer": "18.0.0",
+		"mediawiki/mediawiki-codesniffer": "20.0.0",
 		"jakub-onderka/php-console-highlighter": "0.3.2",
 		"mediawiki/minus-x": "0.3.1",
 		"mediawiki/mediawiki-phan-config": "0.2.0"