fix(ui-shell): a11y issue with side nav item (#11044)

### Related Ticket(s)

Closes #10918

### Description

The bx-side-nav-menu-item tag in UI Shell component throws accessibility error.
 
### Changelog


**Changed**

- Added role attribute value button to the element.


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<img width="1696" alt="Screenshot 2023-10-17 at 3 51 06 PM" src="https://github.com/carbon-design-system/carbon-for-ibm-dotcom/assets/58620134/e4264482-547e-4bc0-bc18-6bb5dde8b785">

packages/carbon-web-components/src/components/ui-shell/side-nav-menu-item.ts

@@ -64,6 +64,13 @@ class BXSideNavMenuItem extends FocusMixin(LitElement) {
     return true;
   }
 
+  connectedCallback() {
+    if (!this.hasAttribute('role')) {
+      this.setAttribute('role', 'button');
+    }
+    super.connectedCallback();
+  }
+
   render() {
     const { active, href, title } = this;
     const classes = classMap({

packages/web-components/tests/snapshots/dds-masthead-composite.md

@@ -86,6 +86,7 @@
     <dds-left-nav-menu-item
       data-autoid="dds--masthead__l0--sidenav--nav0"
       href="https://carbon-design-system.github.io/carbon-for-ibm-dotcom/canary/web-components/foo"
+      role="button"
       title="item-title-foo"
     >
     </dds-left-nav-menu-item>
@@ -106,6 +107,7 @@
     <dds-left-nav-menu-item
       data-autoid="dds--masthead__l0--sidenav--nav1-list0"
       href="https://carbon-design-system.github.io/carbon-for-ibm-dotcom/canary/web-components/bar"
+      role="button"
       title="menu-item-title-bar"
     >
     </dds-left-nav-menu-item>