Display iteration dates in issues filtered search dropdown

What does this MR do and why?

The changes is behind TWO feature flags vue_issues_list and iteration_cadences.

Resolves #343523 (closed).

We are no longer hardcoding start/due dates in title attribute for auto-generated iterations and the iteration dropdown needs to utilize and display start/due dates separately.

⚠ Note that the date format used here does not conform to the upcoming guideline and should be updated #344606.

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Enable the vue issues list and iteration cadences

   Feature.enable(:vue_issues_list)
   Feature.enable(:iteration_cadences)

2. Create a new iteration cadence for a group (ex. gitlab-org). Makes sure to check Automated scheduling (it should be checked by default).

3. Visit any issues list (project or group) and add an iteration filter in the filtered search bar.

MR acceptance checklist

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