Migrate to PHPUnit 12

[srtfisher]

Description

I would like to see this project fully buy into latest version of PHPUnit. This is being started in #1144 with a GitHub workflow update. But we're still requiring yoast/phpunit-polyfills.

Acceptance Criteria

1. Drop yoast/phpunit-polyfills as a dependency.
2. Migrate the tests to PSR-4 format using https://github.com/alleyinteractive/wp-to-psr-4
3. Upgrade mantle-framework/testkit to ^1.0.

Use Case

When a user develops on this plugin they use the latest PHPUnit version available.