Remove Bootstrap 4's Cards components from Epics

Parent epic: &5018 (closed)

With the Bootstrap migration from 3 to 4 (gitlab-foss#45185 (closed)), we introduced app/assets/stylesheets/bootstrap_migration.scss for the rapid integration in gitlab-foss!18232 (merged). Like #284597 (closed), Cards components should be removed from Epics (epic list).