Replace fa-user-secret with GitLab SVG incognito icon

app/assets/stylesheets/framework/header.scss

@@ -235,9 +235,8 @@
         border-top-left-radius: 0;
         border-bottom-left-radius: 0;
 
-        i {
+        svg {
           color: $orange-500;
-          font-size: 20px;
         }
       }
     }

app/views/layouts/header/_default.html.haml

@@ -87,7 +87,7 @@
           - if has_impersonation_link
             %li.nav-item.impersonation.ml-0
               = link_to admin_impersonation_path, class: 'nav-link impersonation-btn', method: :delete, title: _('Stop impersonation'), aria: { label: _('Stop impersonation') }, data: { toggle: 'tooltip', placement: 'bottom', container: 'body', qa_selector: 'stop_impersonation_link' } do
-                = icon('user-secret')
+                = sprite_icon('incognito', size: 18)
           - if header_link?(:sign_in)
             %li.nav-item
               %div

spec/features/admin/admin_users_spec.rb

@@ -353,7 +353,7 @@ RSpec.describe "Admin::Users" do
         it 'sees impersonation log out icon' do
           subject
 
-          icon = first('.fa.fa-user-secret')
+          icon = first('[data-testid="incognito-icon"]')
           expect(icon).not_to be nil
         end