Update GitLab CI to use best practice.

Have updated the pipeline, and have some minor issues:

• PHPStan is listing a number of warnings. The "new static" warnings can be handled with a phpstan.neon config file, but the others will need fixing.
• Tests are failing on future Drupal versions. I think this could be important to fix, especially with 10.2 releasing soon.

I've done some fixing-up of the PHPStan and PHPUnit issues, so everything is passing now.

I want to do some more manual testing before merging. If anyone has time to test MR !18, it would be helpful.

Merge successful.

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