Use the new GitLab CI system

Problem/Motivation

It is a good practice to test each new PR with the new automated GitLab CI tool.

What need to be done:

- Create some basic tests (code validation, linting, deprecations)
- Setup a matrix of testing envs (PHP versions, mysql types)
- Create the GitLab CI files
- Create a new PR and test the CI

Guides and resources:

- GitLab CI: https://docs.gitlab.com/ee/ci
- Drupal.org, GitLab CI documentation: https://www.drupal.org/node/3356364 →
- https://www.drupal.org/project/gitlab_templates →