Port EE changes for mirrors/_instructions.html.haml

What does this MR do?

Port EE changes for mirrors/_instructions.html.haml

Are there points in the code the reviewer needs to double check?

Was this ever mentioning something EE only?

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Conforms to the code review guidelines
  • Has been reviewed by a UX Designer
  • Has been reviewed by a Frontend maintainer
  • Has been reviewed by a Backend maintainer

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/21100