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

spec/helpers/application_helper_spec.rb

@@ -26,7 +26,7 @@ describe ApplicationHelper do
 
   describe '#empty' do
     it "builds an 'i' tag" do
-      expect(empty).to have_css('i.empty.fa.fa-minus')
+      expect(empty).to have_css('i.empty.fa-solid.fa-minus')
     end
   end
 
@@ -39,7 +39,7 @@ describe ApplicationHelper do
 
   describe '#no' do
     it "builds an 'i' tag" do
-      expect(no).to have_css('i.fa.fa-xmark')
+      expect(no).to have_css('i.fa-solid.fa-xmark')
     end
   end
 
@@ -141,7 +141,7 @@ describe ApplicationHelper do
 
   describe '#yes' do
     it "builds an 'i' tag" do
-      expect(yes).to have_css('i.fa.fa-check')
+      expect(yes).to have_css('i.fa-solid.fa-check')
     end
   end
 end