Remove aria-haspopup from dropdowns (#33624)

`aria-haspopup="true"` is really intended to signal that an ARIA `menu` will be opened on activation. as a result, some assistive technologies will announce controls with `aria-haspopup="true"` as a menu or menu item (e.g. JAWS and NVDA).
In addition, `aria-haspopup` seems to trigger a bug in Edge/Narrator where the `aria-expanded` state is not correctly announced at the moment when `aria-haspopup` is present.
This now makes the dropdown button more like a generic disclosure widget control - see also https://www.w3.org/TR/wai-aria-practices-1.2/examples/disclosure/disclosure-navigation.html01

site/content/docs/4.6/components/navs.md

@@ -257,7 +257,7 @@ Add dropdown menus with a little extra HTML and the [dropdowns JavaScript plugin
     <a class="nav-link active" href="#">Active</a>
   </li>
   <li class="nav-item dropdown">
-    <a class="nav-link dropdown-toggle" data-toggle="dropdown" href="#" role="button" aria-haspopup="true" aria-expanded="false">Dropdown</a>
+    <a class="nav-link dropdown-toggle" data-toggle="dropdown" href="#" role="button" aria-expanded="false">Dropdown</a>
     <div class="dropdown-menu">
       <a class="dropdown-item" href="#">Action</a>
       <a class="dropdown-item" href="#">Another action</a>
@@ -283,7 +283,7 @@ Add dropdown menus with a little extra HTML and the [dropdowns JavaScript plugin
     <a class="nav-link active" href="#">Active</a>
   </li>
   <li class="nav-item dropdown">
-    <a class="nav-link dropdown-toggle" data-toggle="dropdown" href="#" role="button" aria-haspopup="true" aria-expanded="false">Dropdown</a>
+    <a class="nav-link dropdown-toggle" data-toggle="dropdown" href="#" role="button" aria-expanded="false">Dropdown</a>
     <div class="dropdown-menu">
       <a class="dropdown-item" href="#">Action</a>
       <a class="dropdown-item" href="#">Another action</a>