Update license_management job definition

What does this MR do?

Update licence_management job definition in docs. With https://gitlab.com/gitlab-org/gitlab-ce/issues/46050 and its MR https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/21853, we can now use directly the new env vars to specify the version of the docker image.

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/46050

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• EE specific content should be in the top level /ee folder
• For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?

See https://docs.gitlab.com/ce/ci/docker/using_docker_images.html#overriding-the-entrypoint-of-an-image for documentation on overriding the entrypoint of an image (I was expecting a syntax closer to the one in K8S).

@axil Could you please take a look, or assign someone else? Thanks

/cc @bikebilly @groulot