Simplify and label date calculation (#16052)

* Name the created after timestamp and remove temporary variables

The logic used to calculate the live at time, the week ago time, then
compare them to find the more recent value.

Move all of this logic to a method (there's a guard to exit if
the setting is nil, so this should be safe).

* Mark method private

This is not part of the public api (for workers, this should be
`#perform` alone). Mark it private

app/workers/broadcasts/send_welcome_notifications_worker.rb

@@ -10,12 +10,14 @@ module Broadcasts
       # and will ultimately be superseded by 7.days.ago when it's larger than feature_live_date.
       return unless Settings::General.welcome_notifications_live_at
 
-      notifications_live_at = Settings::General.welcome_notifications_live_at
-      week_ago = 7.days.ago
-      latest_date = notifications_live_at > week_ago ? notifications_live_at : week_ago
-      User.select(:id).where("created_at > ?", latest_date).find_each do |user|
+      User.select(:id).where("created_at > ?", created_after).find_each do |user|
         Broadcasts::WelcomeNotification::Generator.call(user.id)
       end
     end
+
+    def created_after
+      [Settings::General.welcome_notifications_live_at, 7.days.ago].max
+    end
+    private :created_after
   end
 end