[jaygeorge]

Description of the Problem

Two CP accessibility issues from #15409:

1. The burger menu SVG (burger-menu-no-border.svg) had a hardcoded title id referenced by aria-labelledby. Because the icon is inlined in two places in the header, the page emitted duplicate ARIA ids (duplicate-id-aria).
2. Empty-state screens jumped from the page <h1> to item <h3>s with no <h2>, which axe flags as heading-order (dashboard empty state, collections, taxonomies, navigation, fieldsets, roles, groups, etc.).

What this PR Does

1. Makes the burger SVG decorative (aria-hidden="true") and removes the title / labelledby / role markup. The toggle button already has its own accessible name via aria-label.
2. Gives empty states a proper outline: EmptyStateMenu heading is an h2 (via a new CardPanel level prop), and EmptyStateItem headings default to h3 (overridable with a level prop).
3. Adds a Vitest covering the h2 → h3 hierarchy.

Visual appearance is unchanged (heading styles come from size, not the tag; burger paths are identical).

Fixes #15409

How to Reproduce

1. On 6.x, open /cp/playground (or any CP page) and run:
   document.querySelectorAll('#burger-menu-title').length → > 1
2. Open an empty-state screen (e.g. /cp/roles or /cp/fieldsets with no items) and run:
   [...document.querySelectorAll('h1,h2,h3')].map(h => h.tagName) → H1 then H3s
3. On this branch, hard-reload and repeat:
   • burger id count → 0
   • headings → H1, H2 (menu), then H3s (items)