Clarify iteration rollover is dependent on timezone configuration

What does this MR do and why?

Issue #392405 (closed)

Updates the cadence form and its feature documentation to clarify that issues rollover at midnight in the timezone configured for GitLab instance.

Before	After

How to test the change

1. Visit any group

2. Navigate to the cadence feature page via the sidebar on the left. Issues > Iterations.

3. Click New iterations cadence button in the top right corner.

To use another timezone, you need to update gitlab.yml and restart your GDK.

Open config/gitlab.yml and change the timezone:

    ## Date & Time settings
    # Uncomment and customize if you want to change the default time zone of GitLab application.
    # To see all available zones, run `bundle exec rake time:zones:all RAILS_ENV=production`
-    # time_zone: 'UTC'
+    time_zone: 'Hawaii'

After saving the change, restart the GDK.

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