Add and apply Rails/DurationArithmetic rubocop rule

This rule was added in rubocop-rails 2.13.0. We were already applying it
most of the time.

spec/requests/dashboard/group_supports_spec.rb

@@ -1,7 +1,7 @@
 require "rails_helper"
 
 describe Dashboard::GroupSupports do
-  let(:created_at) { Time.current - 9.days }
+  let(:created_at) { 9.days.ago }
   let(:proposal) { create(:proposal, created_at: created_at, published_at: created_at) }
 
   before do