Update left side nav border UI

What does this MR do?

The left side navigation uses a 2px border (inset shadow), inconsistent with the right issuable sidebar that uses a 1px border.
Same applies to the top-border of the bottom toggle-sidebar-button.

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

Why was this MR needed?

The left side navigation UI borders are not consistent with the right issuable sidebar.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Conform by 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
  • Has been reviewed by a Database specialist
• Conform by the merge request performance guides
• Conform by the style guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them
• Internationalization required/considered
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

Closes #46222 (closed)