Update yoast/phpunit-polyfills requirement from ^3.0 to ^4.0

Updates the requirements on [yoast/phpunit-polyfills](https://github.com/Yoast/PHPUnit-Polyfills) to permit the latest version.
- [Release notes](https://github.com/Yoast/PHPUnit-Polyfills/releases)
- [Changelog](https://github.com/Yoast/PHPUnit-Polyfills/blob/4.x/CHANGELOG.md)
- [Commits](Yoast/PHPUnit-Polyfills@3.0.0...4.0.0)

---
updated-dependencies:
- dependency-name: yoast/phpunit-polyfills
  dependency-type: direct:development
...

Signed-off-by: dependabot[bot] <[email protected]>

composer.json

@@ -12,7 +12,7 @@
         "phpcompatibility/phpcompatibility-wp": "*",
         "squizlabs/php_codesniffer": "3.*",
         "wp-coding-standards/wpcs": "dev-develop",
-        "yoast/phpunit-polyfills": "^3.0",
+        "yoast/phpunit-polyfills": "^4.0",
         "dealerdirect/phpcodesniffer-composer-installer": "^1.0.0",
         "sirbrillig/phpcs-variable-analysis": "^2.11",
         "phpcsstandards/phpcsextra": "^1.1.0",