Coding Standards: Update PHPCS to version 3.8.1.

PHPCS has seen two new releases since the update to WPCS 3.0, with especially the 3.8.0 version containing a huge number of improvements.

References:
* [https://github.com/PHPCSStandards/PHP_CodeSniffer/releases/tag/3.8.0 PHP_CodeSniffer 3.8.0 release notes]
* [https://github.com/PHPCSStandards/PHP_CodeSniffer/releases/tag/3.8.1 PHP_CodeSniffer 3.8.1 release notes]

Follow-up to [56695].

Props jrf, swissspidy.
Fixes #60279.

git-svn-id: https://develop.svn.wordpress.org/trunk@57378 602fd350-edb4-49c9-b593-d223f7449a82

composer.json

@@ -16,7 +16,7 @@
 		"ext-dom": "*"
 	},
 	"require-dev": {
-		"squizlabs/php_codesniffer": "3.7.2",
+		"squizlabs/php_codesniffer": "3.8.1",
 		"wp-coding-standards/wpcs": "~3.0.1",
 		"phpcompatibility/phpcompatibility-wp": "~2.1.3",
 		"yoast/phpunit-polyfills": "^1.1.0"