Make Iteration Cadence Start Date Not Required When Manual

What does this MR do and why?

Start date should not be required when creating a manual iteration cadence

Database Review

Migration Output

UP

Duration: 5.235 ms (from database-lab)

== 20211123182614 MakeIterationCadencesStartDateNullable: migrating ===========
-- change_column_null(:iterations_cadences, :start_date, true)
   -> 0.0039s
== 20211123182614 MakeIterationCadencesStartDateNullable: migrated (0.0039s) ==

DOWN

== 20211123182614 MakeIterationCadencesStartDateNullable: reverting ===========
-- change_column_null(:iterations_cadences, :start_date, false)
   -> 0.0034s
== 20211123182614 MakeIterationCadencesStartDateNullable: reverted (0.0067s) ==

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

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

Related to #342758 (closed)