Change grid classes to utilities to make vertical nav example more resilient to narrow viewports.
/cc @XhmikosR
Fixes #31771 (closed)
Non-whitespace diff: https://github.com/twbs/bootstrap/pull/31785/files?w=1 Preview: https://deploy-preview-31785--twbs-bootstrap.netlify.app/docs/5.0/components/navs/#javascript-behavior