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/tips/_sortable_tip.html.slim

@@ -1,8 +1,8 @@
 - tip = exercise_tip.tip
 .list-group-item.d-block data-tip-id=tip.id data-id=exercise_tip.id
-  span.fa.fa-bars.mr-3
+  span.fa-solid.fa-bars.mr-3
   = tip.to_s
-  a.fa.fa-eye.ml-2 href=tip_path(tip) target='_blank'
-  a.fa.fa-xmark.ml-2.remove-tip href='#'
+  a.fa-regular.fa-eye.ml-2 href=tip_path(tip) target='_blank'
+  a.fa-solid.fa-xmark.ml-2.remove-tip href='#'
   .list-group.nested-sortable-list class="#{exercise_tip.children.present? ? 'mt-3' : ''}"
     = render(partial: 'tips/sortable_tip', collection: exercise_tip.children, as: :exercise_tip)