Fix sass deprecation warning

[wjhdev]

DEPRECATION WARNING: Using / for division is deprecated and will be removed in Dart Sass 2.0.0.

Recommendation: math.div($pixels, $base)

More info and automated migrator: https://sass-lang.com/d/slash-div

  ╷
5 │   @return percentage(($pixels / $base));
  │                       ^^^^^^^^^^^^^^^
  ╵
    src/global/sass/foundations/_grid.scss 5:23  flex()
    stdin 7:18                                   root stylesheet