Update yoast/phpunit-polyfills to ^4.0

Current major; compatible with PHPUnit 9.6 and the WordPress test
framework.

Author: sirreal

composer.json

@@ -32,7 +32,7 @@
 	"require-dev" : {
 		"phpunit/phpunit": "^9.6",
 		"spatie/phpunit-watcher": "^1.23",
-		"yoast/phpunit-polyfills": "^1.0"
+		"yoast/phpunit-polyfills": "^4.0"
 	},
 	"scripts"    : {
 		"test": "phpunit",