remove dropdown.js reliance on roles and fix keyboard navigation

Created by: pvdlg

Per migration documentation in https://github.com/twbs/bootstrap/commit/4d8d8bdab4b0e2931fcf92ce30d772b05a989434:

• Rebuilt dropdown styles and markup to provide easy, built-in support for <a> and <button> based dropdown items.
• Dropdown items now require .dropdown-item.

In addition fix the the space key behavior on firefox for <button> elements: #21159 (closed). Thanks to @RyanThomasMusser for the analysis here. Fixes #21159 (closed).

This PR:

• Remove the dependency to role="menu" or role="listbox" and rely on the class dropdown-menu instead
• Remove the dependency to the a element for the dropdown-item and rely on the class dropdown-item instead
• Remove the necessity to wrap dropdown-item in li elements

With this modification the javascript now matches the markup in the documentation. The keyboard navigation works again now in dropdowns.

Also fixes #21941 (closed) (see comment)