Use `div` instead of `span` in the new dropdown item content wrapper

As noticed here, when user provides custom content to the dropdown item, having div as a content wrapper would be semantically more correct

we seem to have mixed divs and spans content in the dropdown. I guess this is related to custom content. Not a big deal but we can follow up with an issue to ensure that the gl-collapsible-listbox uses divs for content wrappers.