Resolve ""Repository mirroring paused" email link is invalid"

What does this MR do?

From: https://gitlab.com/gitlab-org/gitlab-ee/issues/5740#note_69807313

this seems to have a pretty easy solution looking at https://gitlab.com/gitlab-org/gitlab-ee/blob/master/ee/app/views/notify/mirror_was_hard_failed_email.text.erb#L5 we see that we use project_settings_repository_url(@project) which is what we should use in https://gitlab.com/gitlab-org/gitlab-ee/blob/16eb560738c500abe367a372eff780c310da8d38/ee/app/views/notify/mirror_was_hard_failed_email.html.haml#L8 as well.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• EE specific content should be in the top level /ee folder
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• If paid feature, have we considered GitLab.com plan and how it works for groups and is there a design for promoting it to users who aren't on the correct plan
• End-to-end tests pass (package-qa manual pipeline job)

What are the relevant issue numbers?

Closes #5740 (closed)