Disable start date when iteration cadence is in "manual" mode

What does this MR do and why?

Disable 'Start date' field on manual iteration cadences. The start date of a manual cadence is meaningless, so not required. Also reorder the fields, so the disabled ones are grouped together (see #342758 (comment 744335823))

Screenshots or screen recordings

before	after

How to set up and validate locally

1. Enable the invite modal

   Feature.enable(:iteration_cadences)

2. Visit any group Cadences page such as http://127.0.0.1:3000/groups/flightjs/-/cadences
3. Click the Create cadence button.
4. Uncheck 'Automatic', see disabled field -->

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)