Change top nav menu icon to 'hamburger'

- After feedback we landed on this being the
  preferred look and feel.
- https://gitlab.com/gitlab-org/gitlab/-/issues/332532

app/assets/javascripts/nav/components/top_nav_app.vue

@@ -22,7 +22,7 @@ export default {
   <gl-nav class="navbar-sub-nav">
     <gl-nav-item-dropdown
       :text="navData.activeTitle"
-      icon="dot-grid"
+      icon="hamburger"
       menu-class="gl-mt-3! gl-max-w-none! gl-max-h-none! gl-sm-w-auto! js-top-nav-dropdown-menu"
       toggle-class="top-nav-toggle js-top-nav-dropdown-toggle gl-px-3!"
       no-flip

app/views/layouts/nav/_top_nav.html.haml

@@ -2,7 +2,7 @@
 %ul.list-unstyled.navbar-sub-nav#js-top-nav{ data: { view_model: view_model.to_json } }
   %li
     %a.top-nav-toggle{ href: '#', type: 'button', data: { toggle: "dropdown" } }
-      = sprite_icon('dot-grid', css_class: "dropdown-icon")
+      = sprite_icon('hamburger', css_class: "dropdown-icon")
       = view_model[:activeTitle]
       = sprite_icon('chevron-down')
 

spec/frontend/nav/components/top_nav_app_spec.js

@@ -30,7 +30,7 @@ describe('~/nav/components/top_nav_app.vue', () => {
     it('renders nav item dropdown', () => {
       expect(findNavItemDropdown().attributes('href')).toBeUndefined();
       expect(findNavItemDropdown().attributes()).toMatchObject({
-        icon: 'dot-grid',
+        icon: 'hamburger',
         text: TEST_NAV_DATA.activeTitle,
         'no-flip': '',
       });