Converted merged master's screen-xs-max references

app/assets/stylesheets/framework/banner.scss

@@ -23,7 +23,7 @@
     border-bottom: 1px solid $border-color;
   }
 
-  @media (max-width: $screen-xs-max) {
+  @include media-breakpoint-down(xs) {
     justify-content: center;
     flex-direction: column;
     align-items: center;

app/assets/stylesheets/pages/milestone.scss

@@ -223,7 +223,7 @@
   }
 }
 
-@media (max-width: $screen-xs-max) {
+@include media-breakpoint-down(xs) {
   .milestone-banner-text,
   .milestone-banner-link {
     display: inline;