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/layouts/application.html.slim

@@ -41,7 +41,7 @@ html lang="#{I18n.locale || I18n.default_locale}"
       nav.navbar.navbar-dark.bg-dark.navbar-expand-md.mb-4.py-1 role='navigation'
         .container
           .navbar-brand
-            i.fa.fa-code
+            i.fa-solid.fa-code
             = application_name
           button.navbar-toggler data-target='#navbar-collapse' data-toggle='collapse' type='button' aria-expanded='false' aria-label='Toggle navigation'
             span.navbar-toggler-icon