Rename v4 FontAwesome icons to match v5/v6 name

While FontAwesome provides aliases for all icons, we switch to the new names for consistency. See all changes at:
https://fontawesome.com/docs/web/setup/upgrade/upgrade-from-v4#icons-renamed-since-version-4

app/views/exercises/show.html.slim

@@ -48,7 +48,7 @@ ul.list-unstyled#files
       .card-header role="tab" id="heading"
         a.file-heading.collapsed data-toggle="collapse" data-parent="#files" href=".collapse#{file.id}"
           div.clearfix role="button"
-            i class="fa" aria-hidden="true"
+            i.fa-solid aria-hidden="true"
             span = file.filepath
       .card-collapse.collapse class="collapse#{file.id}" role="tabpanel"
         .card-body