Discrepancy between Bootstrap bold and GitLab bold

As part of the effort to start use GitLab UI classes over Bootstrap classes in gitlab-org, I found that Bootstrap's bold class is "bolder" than GitLab UI's

.gl-font-weight-bold { // GitLab UI
   font-weight: 600;
}

.font-weight-bold { // Bootstrap
   font-weight: 700;
}

Creating this issue to try and harmonize bootstrap’s bold with the design system.

Conversation thread can be found here: https://gitlab.slack.com/archives/C0GQHHPGW/p1589830922147800 (Internal Only)

cc: @ealcantara