Add badge for dependency status

What does this MR do?

Add a README badge for dependency status (covers both Gemfile and package.json)

Are there points in the code the reviewer needs to double check?

This targets the GitHub mirror of gitlab-ce, Gemnasium support for gitlab.com repositories is currently in beta

Why was this MR needed?

It's good to have an overview of outdated dependencies and relevant security issues.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/23599