Update iteration dropdown to use dates

What does this MR do and why?

This MR updates the iteration dropdowns in various places for #349612 (closed).

The UX review for this MR was done in !78039 (merged) (I split the MR to keep the size reviewable).

• Ensure that iteration items are displayed properly without title (the backend change will be made separately to avoid the multi-version compatibility issue - this is really a crucial point)
• Gray out iteration titles as we will be making the attribute optional
• Use iteration dates as the main display attributes

Screenshots or screen recordings

Issue/board sidebar

Notice that the orders in which the same set of iterations are listed are different before vs after. This is because the iterations are grouped by cadences by default. It should NOT be an issue for those users who did not opt in for the iteration cadences trial.

FF	Before	After
:iteration_cadences OFF
:iteration_cadences OFF
:iteration_cadences ON
:iteration_cadences ON

Issue bulk edit sidebar

FF	Before	After
:iteration_cadences OFF
:iteration_cadences OFF
:iteration_cadences ON
:iteration_cadences ON

Board iteration list add dropdown

FF	Before	After
:iteration_cadences OFF
:iteration_cadences ON

How to set up and validate locally

Make sure to validate the changes with and without :iteration_cadences

Feature.enable(:iteration_cadences)

Feature.disable(:iteration_cadences)

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.