Deprecate manual iteration management for automatic iteration cadences

The changes included in this MR have been split off from !82458 (merged) and are behind the feature flag :iteration_cadences.

The documentation and UX review/updates will be done in !82458 (merged). Partially closes #353349 (closed).

What does this MR do and why?

Iteration Cadences feature introduces a container object (iteration cadence) for iterations and is in development behind a feature flag. When the feature flag is enabled, any existing iterations are grouped into a default iteration cadence and the iterations in it can continue to be managed manually i.e., users can add/edit/remove iterations. For newly created iteration cadences, iterations are managed strictly via automated scheduling without any user input. Because iterations only serve as time markers in automatic iteration cadences, we are deprecating the title field and only allowing the description field to be editable.

1. When viewing an iteration, remove Delete from the dropdown menu in the iteration report view if the iteration's cadence is automatic.
2. When editing an iteration, only allow the description field to be updated if the iteration's cadence is automatic.
3. iteration_form.vue is updated to use gl-* forms.
4. Update the UI text Update iteration to Save changes !82458 (comment 874131113)

For manual cadences, iterations should continue to be editable and removable (no change here.)

Screenshots

Iteration edit form (for manual cadence)

Iteration edit form (for automatic cadence)

Iteration report view dropdown

iteration in manual cadence	iteration in automatic cadence

How to test

1. Turn off :iteration_cadences FF via rails console.

2. Create a new group and create some iterations.

3. Turn on :iteration_cadences FF via rails console.

Check that the existing iterations have been grouped into an iteration cadence before proceeding

4. Create a new iteration cadence (only automatic cadences can be created.)

Creating a new automatic iteration cadence	Iteration Cadences List

5. (for automatic cadence) Test the description field can only be edited for iterations.
6. (for automatic cadence) Test that iterations cannot be removed via the top-left dropdown in the report view.
7. (for manual cadence) Test that iterations can be added/edited/removed as before.

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

⚠ there is no way to tell if a cadence is automatic or manual in the UI and this is being addressed in !82458 (merged).