open accordion button has a border of 1px, but a closed one suppresses the border-bottom-width:0. this caused an odd 1px jump on the accordion panel. now, the border width is transitioned at the same pace as the panel opening/closing transition, so no more perceptible jump
Closes #32250 (closed)
Preview: https://deploy-preview-32251--twbs-bootstrap.netlify.app/docs/5.0/components/accordion/