Move sticky toolbar menu into <nav> landmark

Problem/Motivation

• All content should be contained within landmark roles
• The sticky toolbar menu is a navigation menu, so it should live within a Navigation Landmark

Steps to reproduce

1. Go to site to see sidebar navigation
2. Inspect the page and search for CSS class admin-toolbar__sticky-section
3. Verify that the menu contains a "ul" element, but it’s not contained within a "nav" element

Code snippet

<div class="admin-toolbar__sticky-section" style="">
  <div class="admin-toolbar__item toolbar-block" style="">
    <ul class="toolbar-menu" style="">

Screenshot

https://www.drupal.org/files/issues/2023-10-20/sticky-nav.png

Proposed resolution

1. Change sticky toolbar menu from "div" to "nav" element
2. Add "h2" heading with id
3. Add aria-labelledby attribute to "nav" element

<nav class="admin-toolbar__sticky-section" style="" aria-labelledby=”toolbar-menu”>
  <div class="admin-toolbar__item toolbar-block" style="">
    <h2 id=””>Toolbar Menu</h2>
    <ul class="toolbar-menu" style="">