Use dynamic attributes in factories

Static attributes are deprecated since factory bot 4.11; we haven't
upgraded yet but now we're ready to do so:

https://thoughtbot.com/blog/deprecating-static-attributes-in-factory_bot-4-11

spec/factories/notifications.rb

@@ -11,13 +11,13 @@ FactoryBot.define do
   factory :admin_notification do
     title             { |n| "Admin Notification title #{n}" }
     body              { |n| "Admin Notification body #{n}" }
-    link              nil
-    segment_recipient UserSegments::SEGMENTS.sample
-    recipients_count  nil
-    sent_at           nil
+    link              { nil }
+    segment_recipient { UserSegments::SEGMENTS.sample }
+    recipients_count  { nil }
+    sent_at           { nil }
 
     trait :sent do
-      recipients_count 1
+      recipients_count { 1 }
       sent_at { Time.current }
     end
   end