[Rails5] Add `Gitlab.rails5?` method

What does this MR do?

Adds the Gitlab.rails5? method.

Why was this MR needed?

It helps to migrate from the current rails-4.x to the rails-5.0.

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
• Internationalization required/considered
• End-to-end tests pass (package-qa manual pipeline job)

What are the relevant issue numbers?

#14286 (closed) and !12841 (closed)