Improve tests setup - Githubissues

Description

Update the integrations tests and CI process to cover newer PHP versions.

Introduce Yoast/WP-Test-Utils and PHPUnit 9.
Update the prepare test script and bootstrap files.
Fix up incorrect uses of factory property to use a factory() method.
Update deprecated or removed assertions.
Refresh the PHPUnit config.
Identify multisite-specific tests via @group instead of skipping tests.
Apply fixes for running under PHP 8.1 and 8.2, including defining dynamic properties.
Remove a test that wasn't testing anything about CAP.
Update the CI test matrix to cover highest and lowest WP version, and highest and lowest PHP 7.x and 8.x versions.

None.