Introduce GITLAB_EDITION to select the edition (FOSS, EE, or JH)

The following discussion from !57807 (merged) should be addressed:

• @ayufan started a discussion: (+4 comments)

  Does it mean that when developing locally we will run JH always?

  Can we consider that there is only way to operate?

  ce < ee < jh

  Maybe we should also change the FOSS_ONLY variable to a GITLAB_VARIANT=foss/ee/jh, with default being different based on a presence of directory?

  This also means that if we will have to introduce another variant in the future, it will already be covered.