[v0] Back-port Gemnasium 2.x

What does this MR do?

Make Dependency Scanning v0 (used in GitLab 10.7-11.5) use Gemnasium 2.x, now connected to the gemnasium-db repo. See gitlab-org/gitlab#14630 (closed)

Changes

CI configuration

• build Docker images for all versions of GitLab using Dependency Scanning v0, from 10.7 up to 11.5
• run the tests related to the gemnasium plugin/analyzer in its own job, where gemnasium:2.3.0 is the base image

Dockerfile

• build image on top of gemnasium:2.3.0, which already includes the Gemnasium CLI, its vrange library, as well as a clone of gemnasium-db

Gemnasium Ruby class

• file detection based on what gemnasium, gemnasium-maven, and gemnasium-python currently support
• Gemnasium CLI integration
• Gemnasium Docker images integration
• conversion from Dependency Scanning report v2 - what the Gemnasium CLI and images generate

What are the relevant issue numbers?

gitlab-org/gitlab#33321 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added
• [-] Documentation created/updated for GitLab EE, if necessary
• [-] Documentation created/updated for this project, if necessary
• [-] Documentation reviewed by technical writer or follow-up review issue created
• [-] Tests added for this feature/bug
• [-] Job definition updated, if necessary
• Conforms to the code review guidelines
• Security reports checked/validated by reviewer