Show text saying Can't approve if a user can't approve an MR

Proposal

Following on from this internal RFH: https://gitlab.com/gitlab-com/request-for-help/-/issues/3198+

When a user is not allowed to approve an MR, they do not see an Approve button in the MR widget.

Instead of not showing a button, we should display some text that says something like Can't approve, so it doesn't look like a bug with the missing button.

A future iteration could also display the reason why the user can't approve.