After migrating to Bootstrap 5, adjust the grid variables and the bootstrap build to match an 8px grid.

There is already a PR on this subject, but it might be outdated as Bootstrap 5 introduced a new grid breakpoint.

Our current grid is based on the default Bootstrap variables, which typically use a 15px gutter between columns, the idea is to setup a set of grid variables to support the 8pt grid system (more details in the P4 Grid Update doc)

Refer to the bootstrap docs for more info on the grid variables.