Switch to using `data-qa-username` instead of `data-qa-index`

Per reviewer feedback

app/assets/javascripts/admin/users/components/user_actions.vue

@@ -110,7 +110,7 @@ export default {
         :text-sr-only="!showButtonLabels"
         icon="settings"
         data-qa-selector="user_actions_dropdown_toggle"
-        :data-qa-index="user.id"
+        :data-qa-username="user.username"
       >
         <gl-dropdown-section-header>{{
           $options.i18n.userAdministration

qa/qa/page/admin/overview/users/show.rb

@@ -29,7 +29,7 @@ module QA
             end
 
             def open_user_actions_dropdown(user)
-              click_element(:user_actions_dropdown_toggle, index: user.id)
+              click_element(:user_actions_dropdown_toggle, username: user.username)
             end
 
             def click_impersonate_user