Add deprecation alert for manual cadences

What does this MR do and why?

• Group manual cadences in the top in the cadence list
• Show a deprecation warning for manual cadences (in the cadence list and in the update form)
• Remove "automatic" checkbox from the cadence new/edit form

Resolves #353349 (closed).

Screenshots

Cadence list (when including manual cadence(s))

Cadence list (when only including automatic cadence(s))

Cadence create form

Cadence edit form (for manual cadence)

Cadence edit form (for automatic cadence)

How to set up and validate locally

1. Turn on :iteration_cadences FF via rails console.

2. On master branch create a new group with one automatic cadence and one manual cadence with some iterations in it.

3. Test that the manual cadence can be edited and updated to use the automatic mode.

4. Test that only automatic cadences can be created.

5. (for automatic cadences) Test that iterations cannot be individually added.

6. (for manual cadences) Test that iterations can be added as before.

When iteration_cadences FF is turned off, no change should be observed.

MR acceptance checklist

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