Use semantic tokens in app/assets/stylesheets/components/content_editor.scss

See scan results at https://gitlab-org.gitlab.io/frontend/playground/gitlab-code-scanner/?search=check_id%20co%20%22color%22 for app/assets/stylesheets/components/content_editor.scss

This is about the definition of the background color used for selected text in our ProseMirror editor. It's unclear whether we have a token appropriate to this usage yet.