Test gitlab4j-api against multiple version of GitLab

When I start to maintain this project, I faced few problems between to use it with some of GitLab versions. I'm not the only one. See https://github.com/gitlab4j/gitlab4j-api/issues/682#issuecomment-819466002, #658, #681

From my point of view there is a lack of test against multiple versions of GitLab. After some search, I have a plan to work on this issue.

• [ ] Use TestContainer instead of launching a single container (actually GitLab 12.9.2-ce.0)
• [ ] Use JUnit 5 TestTemplate and provide a list of tested versions
• [ ] Update CI accordingly to the change in tests (maybe get rid of Travis CI and use GitHub Action instead)

I saw this as a huge improvement for stability of the project. We can even create a report with which versions of GitLab are supported by a specific version of gitlab4j-api.

I think it's also a mandatory infrastructure before any creation of gitlab4j-api v5