Setup GitLab CI pipelines for phpcs, phpstan, eslint, composer validations

There were three blocked issues to merge this one:
- 🐛 Error: Call to a member function get() on null in Drupal\private_message\Form\PrivateMessageForm->save() (line 481 of modules/contrib/private_message/src/Form/PrivateMessageForm.php). Fixed
- 📌 SQL Error at checkPrivateMessageMemberExists nethod Fixed
- 📌 Add settings.count_method missing schema Fixed
As all of them are fixed/merged already, this one is also merged.

Still, we have eslint, phpcs, phpstan, styleling warnings to fix. All of them to be fixed at:
- 📌 DI problems Needs review
- 📌 Fix the issues reported by phpcs Needs review

Summary of this issue:
- gitlab ci pipelines has been added;
- phpstan configuration has been added;
- DrupalCI (deprecated) tests run for 3.0.x branch have been removed;

Automatically closed - issue fixed for 2 weeks with no activity.