refactor Bootstrap 4 features dropped in Bootstrap 5

I haven't done a full search, but I found usages in some SCSS files referring to the class `media-body`, which has been dropped in BS5, according to this Moodle site: https://moodledev.io/docs/4.5/guides/bs5migration#media

Thus, I suppose some styles are not in effect in Moodle 4.5