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/_collapsed_card.html.slim

@@ -5,7 +5,7 @@
     .card-title.mb-0
       a.collapsed aria-controls="tip-collapse-#{exercise_tip.id}" aria-expanded="false" data-parent="#tips" data-toggle="collapse" href="#tip-collapse-#{exercise_tip.id}"
         .clearfix role="button"
-          i.fa aria-hidden="true"
+          i.fa-solid aria-hidden="true"
           span
             = t('activerecord.models.tip.one')
             =< tip_prefix + index.to_s

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)