[MERGE] web: update bootstrap from 4.1.3 to 4.2.1

Bootstrap just released a brand new version with new components and new possibilities, which are great to integrate in master.

Note: Unfortunately, this also comes with some bug fixes they did not add in a 4.1.4 version, so there is no clean way to backport them in Odoo 12.0 (so if any is required at some point, we will have to use bootstrap_review.scss or anything else to bug fix).

closes odoo/odoo#29949