Update yoast/phpunit-polyfills requirement from ^2.0 to ^3.0

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/3.x/CHANGELOG.md)
- [Commits](Yoast/PHPUnit-Polyfills@2.0.0...3.0.0)

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

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

composer.json

@@ -39,7 +39,7 @@
     "slevomat/coding-standard": "~8.12",
     "squizlabs/php_codesniffer": "^3.2",
     "wp-phpunit/wp-phpunit": "~6.5",
-    "yoast/phpunit-polyfills": "^2.0"
+    "yoast/phpunit-polyfills": "^3.0"
   },
   "suggest": {
     "frontpack/composer-assets-plugin": "Composer plugin for copying of frontend assets into public directory."