Updates the font weight of button styles because of the change to system fonts

Former-commit-id: 4f264c79

app/assets/stylesheets/framework/buttons.scss

 @mixin btn-default {
   border-radius: 3px;
   font-size: $gl-font-size;
-  font-weight: 500;
+  font-weight: 400;
   padding: $gl-vert-padding $gl-btn-padding;
   &:focus,

changelogs/unreleased/update-button-font-weight.yml

+---
+title: Updates the font weight of button styles because of the change to system fonts
+merge_request:
+author: