Make RuboCop aware of Rails concerning and delegate methods

[mattbrictson]

In its default configuration, RuboCop's Lint/UselessAccessModifier rule can get confused by the concerning and delegate constructs in Rails.

Take this code for example,

class User < ApplicationRecord
  concerning :Authentication do
    has_secure_password
    before_save :some_callback_method

    private

    def some_callback_method
      # ...
    end
  end

  private

  def another_private_method
    # ...
  end
end

RuboCop sees two private modifiers and considers this a violation, even though they take place in two different scopes.

This PR fixes this by configuring Lint/UselessAccessModifier to understand Rails-specific syntax, as recommended in the RuboCop docs: https://docs.rubocop.org/rubocop/cops_lint.html#lintuselessaccessmodifier