.gitlab-ci.yml: Use gitlab's builtin License Compliance analyzer

See https://docs.gitlab.com/ee/user/application_security/license_compliance/

From there:

Note: If the license compliance report doesn’t have anything to compare to, no information will be displayed in the merge request area. That is the case when you add the license_management job in your .gitlab-ci.yml for the first time. Consecutive merge requests will have something to compare to and the license compliance report will be shown properly.