Switch GitLab CI configuration to the template developed by the DA

Problem/Motivation

So it turns out that we can't really run the GitLab CI pipeline whenever we want, because the workflow rules defined in the Drupal Spoons template (https://gitlab.com/drupalspoons/composer-plugin/-/blob/master/templates/...) don't allow it.

This means we can't test against arbitrary versions of core and PHP.

Proposed resolution

This problem has been solved in the Drupal Association's GitLab CI configuration templates. Lightning API is already using those templates to great effect, and indeed, it also includes the addition of ext-sodium that Simple OAuth depends on.

Remaining tasks

Replace .gitlab-ci.yml with the one used by Lightning API. It should work as a drop-in replacement and will allow the pipeline to be run at any time.