Migrate gitlab api from 5 to 6

[alecharp]

This provides the migration of the GitLab API from version 5 to version 6.
Requires jenkinsci/gitlab-api-plugin#121.

Testing done

Ran mvn verify locally without any problem.

Submitter checklist

• Make sure you are opening from a topic/feature/bugfix branch (right side) and not your main branch!
• Ensure that the pull request title represents the desired changelog entry
• 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 demonstrate the feature works or the issue is fixed

[alecharp]

I was able to use the latest build of the latest commit to build https://gitlab.com/MarkEWaite/hugo-experiment without problem.

I used a multibranch pipeline, set GitLab Project source, didn't use any credentials, used MarkEWaite user, it successfully listed all repository. On branch scanning, it discovers 2 MR and a branch, built all of that. It also renamed the project to use GitLab user and repository name.

[MarkEWaite]

I've tested the incremental build of this pull request with the latest incremental build from:

• Update gitlab4j to 6.1.0, update Jenkins BOM, require Jenkins 2.504.3 gitlab-api-plugin#121

Due to the breaking changes in the transition from GitLab API 5.8.x to GitLab API 6.x, we need to plan a coordinated release of this pull request and other pull requests:

• Update gitlab4j to 6.1.0, update Jenkins BOM, require Jenkins 2.504.3 gitlab-api-plugin#121
• Update gitlab-api dependency gitlab-oauth-plugin#194
• Use GitLab API plugin 6.1.0 gitlab-logo-plugin#131

If a user installs the new release of GitLab API plugin that includes GitLab API 6.x, without updating any of the other installed plugins, those installed plugins will likely be broken and report errors.

I will be out of the office from Oct 27 - Nov 7, so won't be able to help coordinate the release of those plugins until after I return. Maintainers of those plugins need to all agree that they will release a new version soon after the release of the API plugin pull request.