Support PHPUnit 11 in Drupal 11

Problem/Motivation

PHPUnit 11 was released on February 2, 2024, supporting PHP 8.2 and above. Drupal 11 is still using PHPUnit 10.

Proposed resolution

• Upgrade Drupal 11.x to PHPUnit 11 in this issue, retaining the possibility to downgrade to PHPUnit 10 via Composer if necessary.
• Ignore PHPUnit's own deprecations for the moment, leveraging on the possibility to not opting-in via the --fail-on-phpunit-deprecation CLI toggle introduced in PHPUnit 11.3
• Follow up separately on the fixes for PHPUnit 11 deprecations, the largest of which will be converting the test codebase from using PHPUnit legacy annotations to using the corresponding attributes instead.

Remaining tasks

Follow ups