Define caret after dropdown-toggle the same way for Bootstrap3 as in Bootstrap5
This is a forward compatibility change, probably futuer versions of Bootstrap will behave the same. Many thanks to Monika Andergassen for the suggestion.
in essence, this change switches from CSS icons to <adp:icon> since in default setup (when no themes/icon sets are installed) the CSS based rendering was bad. Using <adp:icon> offers more flexibility and tailorability.