[#5976] Warn for Rails Cops

[koic]

This PR warns for Rails Cops.

The following warning is displayed when the next released rubocop gem is installed.

# The following is an example for operation check.
% rake build
Rails cops have been removed from RuboCop 0.71. Use the `rubocop-rails`
gem instead.

Put this in your `Gemfile`.

gem 'rubocop-rails'

And then execute:

$ bundle install

Put this into your `.rubocop.yml`.

require: rubocop-rails

More information: https://github.com/rubocop-hq/rubocop-rails
    exists /Users/koic/src/github.com/rubocop-hq/rubocop
Successfully installed rubocop-0.70.0
1 gem installed

Based on the migration of RuboCop Performance I am considering the following transition schedule.

• 0.70 ... Current version
• Next release ... Display a warning and prepare a transition period for users
• 0.72 ... Remove the Rails cops from RuboCop Core

#6845 (comment)

---

Before submitting the PR make sure the following are checked:

• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Added an entry to the Changelog if the new code introduces user-observable changes. See changelog entry format.
• The PR relates to only one subject with a clear title
  and description in grammatically correct, complete sentences.
• Run bundle exec rake default. It executes all tests and RuboCop for itself, and generates the documentation.

[bbatsov]

Thanks!