Filter by Iterations within the filter bar

Problem to solve

The filter bar supports filtering by one milestone. Given there are now Iterations, the filtering on issue and merge request lists, issue boards, etc. should provide the ability to filter by iterations.

Proposal

1. You should be able to filter by Milestone: ... and Iteration: ...

Context

This is a follow on from the initial MVC for adding Iterations - #35290 (closed)

Feature Flag

Introduced behind a feature flag: :filter_bar_iterations

Release Notes

description: As part of our ongoing efforts to improve using timeboxes to plan and track work within GitLab, you can now filter issue lists and boards by Iteration.

documentation_url: https://docs.gitlab.com/ee/user/group/iterations/

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