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/assets/javascripts/exercise_collections.js.erb

@@ -172,7 +172,7 @@ $(document).on('turbolinks:load', function() {
         if (collectionExercises.indexOf(exercise.id) === -1) {
           // only add exercises that are not already contained in the collection
           var template = '<tr data-id="' + exercise.id + '">' +
-            '<td><span class="fa fa-bars"></span></td>' +
+            '<td><span class="fa-solid fa-bars"></span></td>' +
             '<td>' + exercise.title + '</td>' +
             '<td><a href="/exercises/' + exercise.id + '"><%= I18n.t('shared.show') %></td>' +
             '<td><a class="remove-exercise" href="#"><%= I18n.t('shared.destroy') %></td></tr>';