Upgrade boost to use Bootstrap 4.6.0

Use 2 Moodle instances, one based on the integration branch including this patch, one on latests master. In both instances take these steps: Testing scenario 1: Test if all Boostrap JavaScript works Login as Admin Create a course C1 Restore the attached backup to course C1 Compare the restored book page by page with the other Moodle instance Verify  the book content looks the same on both instances Verify the JS functionality works for: Alerts (dismiss), Buttons (remember state), Carousel (slide), Dropdown (show menu), Popover (show popover). Tooltips (show tooltip) Access to your apache logs (they use to be placed at /var/log/apache2/error.log) and check that there are no sass parse errors there. Testing scenario 2: Check logs Login as Admin Navigate to Site Administration -> Development -> Debugging Choose Developer.. at Debug messages Tick the Display debug messages checkbox Open a console showing your web server / PHP log files (they use to be placed at /var/log/apache2/access.log and /var/log/apache2/error.log) Navigate to Site Administration -> Development -> Purge Caches Navigate to the Site Homepage Wait for the page to load and rebuild caches Check  log files (access.log and error.log) doesn't contain errors.