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/_export_actions.html.slim

@@ -1,14 +1,14 @@
 - if error
     = button_tag type: 'button', class:'btn btn-primary pull-right export-button export-retry-button', data: {exercise_id: exercise.id} do
-      i.fa.fa-refresh.confirm-icon
+      i.fa-solid.fa-arrows-rotate.confirm-icon
       = t('exercises.export_codeharbor.buttons.retry')
 - else
   - unless exported
     - if !exercise_found || update_right
       = button_tag type: 'button', class:'btn btn-primary pull-right export-action export-button', data: {exercise_id: exercise.id} do
-        i.fa.fa-check.confirm-icon
+        i.fa-solid.fa-check.confirm-icon
         = t('exercises.export_codeharbor.buttons.export')
 
 = button_tag type: 'submit', class:'btn btn-secondary pull-right export-button', data: {dismiss: 'modal'} do
-  i.fa.fa-remove.abort-icon
+  i.fa-solid.fa-xmark.abort-icon
   = exported ? t('exercises.export_codeharbor.buttons.close') : t('exercises.export_codeharbor.buttons.abort')