Fix cadence roll over issues checkbox

What does this MR do and why?

Add missing field to a form post. Now we actually send "Roll over issues" checkbox value when submitting the iteration cadence form

How to set up and validate locally

1. Enable the feature flag

   Feature.enable(:iteration_cadences)

2. Visit any group page, go to Issues > Iterations > Create Iteration Cadence (/groups/gitlab-org/-/cadences/new)
3. Fill out form, check "Roll over issues" checkbox
4. Confirm it is actually checked after saving

Screen_Recording_2021-11-19_at_9.01.40_am

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.