Set up GitlabCI for the branch 8.x-4.x and fix the issues reported during the pipeline execution

It looks like it's working now: https://git.drupalcode.org/project/copyscape/-/pipelines/455440

I'll open a new ticket for fixing the PHPCS / CSPELL / ESLINT issues...

Merged, I allowed some tests to fail in order to merge the MR to the main branch. I'll fix that in another issue.