Bundler 2 support

What does this MR do?

Make the analyzer compatible with Ruby projects using Bundler 2:

• upgrade to bundler 2.0.1
• upgrade to bundler-audit 0.6.1

The version of Bundler is a Docker build argument.

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ee/issues/9590

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 Depends on https://gitlab.com/gitlab-org/security-products/release/issues/62
• Job definition updated, if necessary
  • Auto-DevOps template
  • Job definition example
  • CI Templates
• Conforms to the code review guidelines
• Conforms to the Go guidelines
• Security reports checked/validated by reviewer