Replace fixed "bolder" value in reboot with available variable "font-weight-bolder"

Created by: gijsroge

In _reboot.scss#L164

b,
strong {
  font-weight: bolder; // Add the correct font weight in Chrome, Edge, and Safari
}

we have a fixed value bolder. In my opinion this should be replaced with the available variable $font-weight-bolder in _variables.scss#L291

Useful if you want to use a fixed bold value for <strong> & <b> tags.

Thoughts? I can submit a PR if needed.