Show text saying `Can't approve` if a user can't approve an MR
<!--IssueSummary start--> <details> <summary> Everyone can contribute. [Help move this issue forward](https://handbook.gitlab.com/handbook/marketing/developer-relations/contributor-success/community-contributors-workflows/#contributor-links) while earning points, leveling up and collecting rewards. </summary> - [Close this issue](https://contributors.gitlab.com/manage-issue?action=close&projectId=278964&issueIid=561994) </details> <!--IssueSummary end--> ### 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_.
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