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