Update phpunit to fix security advisory error (PKSA-z3gr-8qht-p93v)

[JanJakes]

Running composer install fails with the following error:

Loading composer repositories with package information
Updating dependencies
Your requirements could not be resolved to an installable set of packages.

  Problem 1
    - Root composer.json requires phpunit/phpunit 8.5.38 (exact version match: 8.5.38 or 8.5.38.0), found phpunit/phpunit[8.5.38] but these were not loaded, because they are affected by security advisories ("PKSA-z3gr-8qht-p93v"). Go to https://packagist.org/security-advisories/ to find advisory details. To ignore the advisories, add them to the audit "ignore" config. To turn the feature off entirely, you can set "block-insecure" to false in your "audit" config.
  Problem 2
    - Root composer.json requires yoast/phpunit-polyfills 2.0.0 -> satisfiable by yoast/phpunit-polyfills[2.0.0].
    - yoast/phpunit-polyfills 2.0.0 requires phpunit/phpunit ^5.7.21 || ^6.0 || ^7.0 || ^8.0 || ^9.0 || ^10.0 -> found phpunit/phpunit[5.7.21, ..., 5.7.27, 6.0.0, ..., 6.5.14, 7.0.0, ..., 7.5.20, 8.0.0, ..., 8.5.52, 9.0.0, ..., 9.6.34, 10.0.0, ..., 10.5.63] but these were not loaded, because they are affected by security advisories ("PKSA-z3gr-8qht-p93v"). Go to https://packagist.org/security-advisories/ to find advisory details. To ignore the advisories, add them to the audit "ignore" config. To turn the feature off entirely, you can set "block-insecure" to false in your "audit" config.

This is due to a security advisory GHSA-vvj3-c3rp-c85p.

It only affects PHPUnit, updating it is an easy fix.

[JanJakes]

I'm going to go ahead and merge so that the CI doesn't fail. This only affects a dev dependency (PHPUnit).