Adds support for legacy artifacts

To allow running the db testing job for old GitLab versions, the testing framework needs to support the old artifact format.

See:

• https://gitlab.slack.com/archives/C3NBYFJ6N/p1718787059744959
• https://gitlab.com/gitlab-org/security/gitlab/-/merge_requests/4189#note_1957531337

Related #154 (closed)