Add Gitlab CI

@BenStallings thanks for getting this issue created, and for your work on it. I was able to get the errors resolved, and am working on the deprecation warnings. I not sure the specifics of why you set this to Needs Work, but if you can give the revised MR a fresh look, maybe we can get this merged in.

I've added back the "previous major" test. I added sections for the composer and phpunit tests for previous major to explicitly set the PHP version. These should be removed after Drupal 11 becomes current.

Moving to RTBC, also creating some follow on issues.

@lostcarpark thanks very much for your work on this. I've merged this in.

I see that you've also created issues for linting errors being thrown by Gitlab CI, I'll get on those next.

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