build: Updating mediawiki/mediawiki-codesniffer to 21.0.0

The following sniffs now pass and were enabled:
* Squiz.Scope.MemberVarScope.Missing

Change-Id: I2eb471b203d176161b7b3660fe73970ece4e8adc

.phpcs.xml

@@ -1,7 +1,6 @@
 <?xml version="1.0"?>
 <ruleset>
 	<rule ref="./vendor/mediawiki/mediawiki-codesniffer/MediaWiki">
-		<exclude name="Squiz.Scope.MemberVarScope.Missing"/>
 		<exclude name="Squiz.Scope.MethodScope.Missing"/>
 	</rule>
 	<file>.</file>

composer.json

@@ -1,7 +1,7 @@
 {
 	"require-dev": {
 		"jakub-onderka/php-parallel-lint": "1.0.0",
-		"mediawiki/mediawiki-codesniffer": "20.0.0",
+		"mediawiki/mediawiki-codesniffer": "21.0.0",
 		"jakub-onderka/php-console-highlighter": "0.3.2",
 		"mediawiki/minus-x": "0.3.1",
 		"mediawiki/mediawiki-phan-config": "0.3.0"