Attempt to fix gitlab tests flakiness

[raul-arabaolaza]

I detected most of the times flakiness was caused by a lack of resources in the Gitlab container, as a result many API calls at random returned a 502 error code and an HTML page asking for the user to refresh...

I have added a custom configuration to the Gitlab fixture which should help in resource constrained environments and increased the request timeout. Also not it uses the readiness checks instead of html web scrapping.

Also some small code changes here and there like closing the gitlab4j clients or slighly reduce the scope of some tests.

Testing done

More than 50 local runs once I got rid of flakiness, also some runs in CloudBees proprietary builders, I also plan to have several runs on this PR builder before moving out of draft status

### Submitter checklist
- [x] Make sure you are opening from a **topic/feature/bugfix branch** (right side) and not your main branch!
- [x] Ensure that the pull request title represents the desired changelog entry
- [x] Please describe what you did
- [ ] Link to relevant issues in GitHub or Jira
- [ ] Link to relevant pull requests, esp. upstream and downstream changes
- [ ] Ensure you have provided tests - that demonstrates feature works or fixes the issue