Update the version support for reply_by_email.md

doc/administration/reply_by_email.md

@@ -3,6 +3,8 @@
 GitLab can be set up to allow users to comment on issues and merge requests by
 replying to notification emails.
 
+Reply by email was added in [GitLab 8.0](https://gitlab.com/gitlab-org/gitlab-foss/-/merge_requests/1173). Please make sure to update to this version.
+
 ## Requirement
 
 Make sure [incoming email](incoming_email.md) is set up.