Set up GitLab CI

Problem/Motivation

Since Jun 2023, Drupal.org enabled GitLab CI for testing purposes. GitLab CI is now preferred over DrupalCI for testing.
This issue is to track progress towards getting a .gitlab-ci.yml file so that we can migrate away from the deprecated DrupalCI to GitLab CI.
GitLab CI documentation: https://www.drupal.org/about/core/blog/drupal-cores-gitlab-ci-testing-is...

Benefits:
1) Testing is about five times faster
@see: https://www.drupal.org/about/core/blog/drupal-cores-gitlab-ci-testing-is...
2) Ability to configure tests in a more obvious and flexible way.
3) Great UI!