Fix (un)approver names not being shown in plaintext emails

app/views/notify/approved_merge_request_email.text.haml

-Merge Request #{@merge_request.to_reference} was approved by #{@approved_by_user}
+Merge Request #{@merge_request.to_reference} was approved by #{@approved_by.name}
 
 Merge Request url: #{project_merge_request_url(@merge_request.target_project, @merge_request)}
 

app/views/notify/unapproved_merge_request_email.text.haml

-Merge Request #{@merge_request.to_reference} was unapproved by #{@unapproved_by_user}
+Merge Request #{@merge_request.to_reference} was unapproved by #{@unapproved_by.name}
 
 Merge Request url: #{project_merge_request_url(@merge_request.target_project, @merge_request)}
 

changelogs/unreleased-ee/3924-approver-emails-text.yml

+---
+title: Fix (un)approver names not being shown in plaintext emails
+merge_request: 3266
+author:
+type: fixed