Introduce support for PHPUnit 14

Problem/Motivation

PHPUnit 14 is due out by February 5, 2027.

One important innovation this release brings is the introduction of native parallel test execution.

Proposed resolution

Adopt PHPUnit 14.

Remaining tasks

Issues tagged 'PHPUnit 14' - https://www.drupal.org/project/issues/search/drupal?text=&assigned=&subm...

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet