fix(bx-header-menu): add button role (#10440)

### Related Ticket(s)

Fixes #10392

### Description

This PR adds `role="button"` to the `<a>` tags inside the L0 menu. This improves accessibility by identifying these elements are acting as buttons. 

> The button role identifies an element as a button to assistive technology such as screen readers. A button is a widget used to perform actions such as submitting a form, opening a dialog, canceling an action, or performing a command such as inserting a new record or displaying information.

### Changelog

**New**

- Markup for `<a>` tags in dropdown menu items now includes `role="button"`.

packages/carbon-web-components/src/components/ui-shell/header-menu.ts

@@ -128,6 +128,7 @@ class BXHeaderMenu extends HostListenerMixin(FocusMixin(LitElement)) {
     return html`
       <a
         part="trigger"
+        role="button"
         tabindex="0"
         class="${prefix}--header__menu-item ${prefix}--header__menu-title"
         href="javascript:void 0"