Resolve "Follow-up from "Keep visual edge with icons and chevron""

What does this MR do and why?

Adds aria-hidden="true" to the <hr> tags of the super sidebar, as discussed in #406547 (closed).

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #406547 (closed)