Warning about non standard 'zoom' CSS property in the browser console

The following warning message is logged in the browser console when accessing XWiki pages:

This page uses the non standard property “zoom”. Consider using calc() in the relevant property values, or using “transform” along with “transform-origin: 0 0”.

This comes from Bootstrap (3.4.1), see https://github.com/twbs/bootstrap/blob/v3.4.1/less/media.less#L13 . This has been fixed with https://github.com/twbs/bootstrap/pull/18682 so we need to upgrade to Bootstrap 4.x (XWIKI-14639).