Set the issuable sidebar to remain collapsed on mobile devices

What does this MR do?

Makes the issuable sidebar to remain collapsed on mobile screens (xs breakpoint)

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

closes #28086 (closed)