Add Automated testing with GitLab CI for ATK

It would be great to have automatic GitLab CI tests for ATK.

Helpful documentation on the topic: Documentation on Drupal.org: GitLab CI → .

The config file template:
https://git.drupalcode.org/project/gitlab_templates/-/blob/main/gitlab-c...

... examples from the DO project base:
https://git.drupalcode.org/search?group_id=2&scope=blobs&search=Gitlab-c...

Relevant example: 📌 Configure GitLab CI Fixed , for the YouTube Field → module, which seems to have a reasonably good user base and pipeline page, see:
https://git.drupalcode.org/project/youtube/-/pipelines

See code file: https://git.drupalcode.org/project/youtube/-/blob/2.x/.gitlab-ci.yml

Work expected:

Use GitLab CI pipelines for automated testing

1. The "first step is to add a .gitlab-ci.yml file to the root of the project."
2. Configure Automated testing pipelines on GitLabCI at:
   https://git.drupalcode.org/project/automated_testing_kit/-/pipelines

This might require that a 2.x dev branch gets created, which would also make development easier, for example when creating MR's.

Currently, the only option for new issues is to select 2.0.0, and manually target MR's at release/2.1, which is not the standard set up. Having a 2.x dev branch would automate these steps.

(Issue text copied and modified from 📌 Automated testing: Configure GitLab CI Active .)