Enable GitLab CI testing

Added the .gitlab-ci.yml from the template and we have our The first pipeline run. It was not very sucessful, as six of the seven jobs failed :-) The only one that passed was variable-checks which just echos some system variables.

The Composer job failed with a surprising error that I don't understand:

$ touch $_WEB_ROOT/core/.env
Uploading artifacts for successful job 
Uploading artifacts...
/builds/project/scheduler: found 68462 matching files and directories 
.git: excluded 1 files 
.git/**/*: excluded 105 files 
ERROR: Uploading artifacts as "archive" to coordinator... 413 Request Entity Too Large id=44659 responseStatus=413 Request Entity Too Large status=413 token=64_A21Dw
FATAL: too large 
Cleaning up project directory and file based variables 00:00 
ERROR: Job failed: command terminated with exit code 1

Everything else failed for various reasons, for example vendor/bin/phpcs: No such file or directory and error Directory "/builds/project/scheduler/web/core" doesn't exist ... most likely due to this first step failing.

Also there appears to be only one dependency link. I'm not sure what that means though. If the composer job is critical, then maybe all the other jobs should depend on it?

That's an improvement. The subsequent jobs are skipped if the Composer job fails

https://git.drupalcode.org/project/gitlab_templates/-/merge_requests/11 has been committed, so Scheduler's modification is now removed.

Would be useful to be able to test the GitLab-CI pipelines without having to push to a MR and have all the noise above.

#3358806: Rename phpcs.xml.dist to phpcs.xml in preparation for GitLabCI fixes the PHPCS problem.

Test via patch

Gitlab testing is now fully implemented for Scheduler 2.x and 8.x-1.x so this issue can be closed.

#3387331: Add GitLab CI testing on 7.x is also complete.