RuboCop Rails supports Rails 4 or higher

[koic]

This PR drops Rails 3 support.

RuboCop core was supported Rails 4+.
https://github.com/rubocop-hq/rubocop/tree/v0.71.0#compatibility

The Rails cops support the following versions:

• Rails 4.0+

It conforms to the expected behavior of RuboCop core originally.

And Rails is maintained at 4.2.Z or higher.

4 Severe Security Issues
For severe security issues all releases in the current major series, and
also the last major release series will receive patches and new
versions. The classification of the security issue is judged by the core
team.

Currently included series: 5.2.Z, 5.1.Z, 5.0.Z, 4.2.Z.

https://guides.rubyonrails.org/maintenance_policy.html

The following is the RuboCop Rails and Rails 4 use case.
#68

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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.