Prevent feature flag in ActiveRecord scope

What does this MR do and why?

Adds a cop to stop the use of feature flag in an ActiveRecord scope.

Having a feature flag in a scope makes the scope behaviour uncertain. Given that a scope can be used in many places, such as through a class method Project.scope, or an instance method project.scope, a feature flag in a scope could lead to unexpected behaviour where the scope is used.

A preferred alternative is to define 2 separate scope implementations, each doing one thing. The caller of the scope can toggle between the 2 scopes using a feature flag.

    # bad:
    #   class Meal < ApplicationRecord
    #     scope: with_fruits, -> do
    #       if Feature.enabled?(:with_apples)
    #         include(:apples)
    #       else
    #         include(:oranges)
    #       end
    #     end
    #   end
    #
    # good:
    #   class Meal < ApplicationRecord
    #     scope: with_apples, -> { include(:apples) }
    #     scope: with_oranges, -> { include(:oranges) }
    #   end
    #
    #   if Feature.enabled?(:with_apples)
    #     Meal.with_apples
    #   else
    #     Meal.with_oranges
    #   end

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