Use a fixed start date for cadence update spec

The factory sets the start date based on a sequence. This can cause the
spec to fail because that date could be the same date that we're
trying to update to which means it wouldn't be changed.

ee/spec/services/iterations/cadences/update_service_spec.rb

@@ -7,7 +7,7 @@ RSpec.describe Iterations::Cadences::UpdateService do
 
   let_it_be(:group, refind: true) { create(:group) }
   let_it_be(:user) { create(:user) }
-  let_it_be(:iteration_cadence, refind: true) { create(:iterations_cadence, group: group, duration_in_weeks: 1, iterations_in_advance: 2) }
+  let_it_be(:iteration_cadence, refind: true) { create(:iterations_cadence, group: group, start_date: Date.today, duration_in_weeks: 1, iterations_in_advance: 2) }
 
   let(:params) do
     {