Always show UTC dates for iteration start/due dates

What does this MR do?

Iteration start/due dates don't have a time - they are just a date string yyyy-mm-dd. This meant users with subzero timezones (e.g. US Pacific time) got a timezone-adjusted date showing the previous day.

This adds the utc parameter and passes it from places in Iterations.

How to test

For dev, requires the group_iterations feature flag, an iteration created, and local timezone set to less than UTC

bin/rspec ee/spec/features/groups/iterations/user_views_iteration_spec.rb

diff --git a/ee/spec/features/groups/iterations/user_views_iteration_spec.rb b/ee/spec/features/groups/iterations/user_views_iteration_spec.rb
index d92ff13b15f..9b834e23eaf 100644
--- a/ee/spec/features/groups/iterations/user_views_iteration_spec.rb
+++ b/ee/spec/features/groups/iterations/user_views_iteration_spec.rb
@@ -18,6 +18,7 @@ RSpec.describe 'User views iteration' do
       end
 
       it 'shows iteration info and dates' do
+        live_debug
         expect(page).to have_content(iteration.title)
         expect(page).to have_content(iteration.description)
         expect(page).to have_content(iteration.start_date.strftime('%b %-d, %Y'))

Screenshots

1. Set system timezone to somewhere on the western half of the world
2. Load iteration
3. Compare date in graphql response with displayed date

Does this MR meet the acceptance criteria?

Closes #227286 (closed)