[Rails5] Add Gemfile.rails5

What does this MR do?

1. Adds Gemfile.rails5 file to generate Gemfile.rails5.lock and keep some gems separated.
2. Modifies Gemfile to support two different versions of gems (for Rails 4 and Rails 5).

The standard way to bundle gems fro the current version (rails 4) is still:

bundle install 

To bundle gems for rails 5 a developer should run:

BUNDLE_GEMFILE=Gemfile.rails5 bundle install

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)