Design: Popover for the merge request assignees that shows "cannot merge"

Following up from #416174 (closed) and from the slack thread https://gitlab.slack.com/archives/C01EMBKS5DW/p1687877682792839

The problem

When hovering over users on the assignees section of merge requests, the user popover no longer appears. This makes it more difficult to gather information about the MR author and determine whether they are a GitLab team member or a member of the community when receiving a review request.

Investigating further, we indeed removed the popover because of the need to be able to show the cannot merge tooltip. We only show the user profile tooltip in the title area of the merge request

A solution find from @kushalpandya was to expose Cannot merge information within the popover:

UI Refinement

Here is the figma link if anyone would like to contribute: https://www.figma.com/file/4KVQUkUDoZilkO4D1o1bvw/Merge-request?type=design&node-id=5602%3A445838&mode=design&t=h69CdMtMFyE6DR8K-1