.gitlab-ci.yml: Move from license-management to license-scanning
The former is removed in gitlab 13.0. Effective now.
More info at https://docs.gitlab.com/ee/user/compliance/license_compliance/#migration-from-license_management-to-license_scanning
Failing build will be like: https://gitlab.com/BuildStream/buildstream/-/jobs/544701397#L521
Edited by Benjamin Schubert