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/_tips_content.html.slim

@@ -7,7 +7,7 @@
 
 #tips.card.mt-2 role="tab" style="display: block;"
   .card-header.py-2
-    i.fa.fa-lightbulb
+    i.fa-solid.fa-lightbulb
     = t('exercises.implement.tips.heading')
   .card-body.text-dark.bg-white.p-2
     = render(partial: 'tips/collapsed_card', collection: @tips, as: :exercise_tip, locals: { tip_prefix: '' })