Apply vertical File Tree layout on larger viewports

Issue: https://gitlab.com/gitlab-org/gitlab/-/issues/345068

app/assets/stylesheets/framework/diffs.scss

@@ -1190,7 +1190,7 @@ table.code {
   }
 }
 
-@media (max-width: map-get($grid-breakpoints, md)-1) {
+@media (max-width: map-get($grid-breakpoints, lg)-1) {
   .diffs .files {
     @include fixed-width-container;
     flex-direction: column;

app/assets/stylesheets/page_bundles/merge_requests.scss

@@ -97,7 +97,7 @@ $tabs-holder-z-index: 250;
   line-height: 0;
 }
 
-@media (max-width: map-get($grid-breakpoints, md)-1) {
+@media (max-width: map-get($grid-breakpoints, lg)-1) {
   .diffs .files {
     .diff-tree-list {
       position: relative;