Add "Main navigation" accessible name to sidebar navigation

What does this MR do and why?

Add "Main navigation" accessible name to sidebar navigation as per https://www.w3.org/WAI/ARIA/apg/practices/names-and-descriptions/

navigation Recommended

Helps screen reader users understand the context and purpose of the navigation landmark.

Use aria-labelledby if a visible label is present, otherwise use aria-label.

See the Navigation Landmark section

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Enable super sidebar (if not currently on)
   1. In rails c enable feature flag Feature.enable(:super_sidebar_nav)
   2. Toggle on "new navigation" in user menu
2. Inspect super sidebar accessibility tree
3. <nav> should have accessible name "Main navigation"

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 #394983 (closed)