[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)