fix(ui-shell): remove menubar role

[tw15egan]

Closes #7377

Removes role based on @dakahn 's comments

Ah! I figured it out. We've got role="menubar" on the <ul> itself in horizontal mode (not mobile, in a dialog). In AIW's eyes this negates any semantics we get from that <ul> -- thus the warning.

Changelog

Removed

• removes role="menubar" from HeaderNavigation

Testing / Reviewing

Ensure there are no a11y violations

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