Switch dropdowns to use semantic <ul> and <li> markup for accessibility

Bootstrap V4 is moving back to recommending semantic markup for their dropdown menus.

Wrapping the dropdown items (and other sub-components) inside `<li>`  elements, and setting the dropdown menu container to a `<ul>`

https://github.com/twbs/bootstrap/pull/28591
https://github.com/twbs/bootstrap/issues/25728

We should update these components to use the new recommended markup.
