Renders assignee list as a vertical list in merge requests

What does this MR do and why?

Changes how the assignee list is rendered in merge requests to be vertical in order to support the requires attention feature.

Closes #343324 (closed)

Screenshots or screen recordings

How to set up and validate locally

• Enable the mr_attention_requests feature flag

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. Closes #343324 (closed)