We need a branch that supports JDK 8

[otary]

I found that the Gitlab main branch has been upgraded to version 6. x, which supports JDK11+and cannot be adapted to the project. Can you create a branch that supports JDK8?

[jmini]

In my opinion you should have a way to move out of Java 8…

That said, my idea described in #1067 would be to:

• gitlab4j-models --> go back to java 8 (I think this is quite easy to do)
• Introduce a gitlab4j-api-jersey2 which should be java 8 as well and that should be a copy of gitlab4j-api (+ a revert of the java 11 changes to use jersey2 and the javax namespace instead of jersey3)

But this is quite some work, and I do not really have the capacity to do this.

And I am open for other proposals.