fix: Rename unread_since scope (#7267)

- This renames the unread_since scope to created_since, which more accurately describes what the scope returns.
- The EXTRACT(EPOCH FROM created_at) > (?) clause was also simplified and rewritten as created_at > ?, which is equivalent:

app/models/conversation.rb

@@ -166,11 +166,11 @@ class Conversation < ApplicationRecord
   end
 
   def unread_messages
-    messages.unread_since(agent_last_seen_at)
+    messages.created_since(agent_last_seen_at)
   end
 
   def unread_incoming_messages
-    messages.incoming.unread_since(agent_last_seen_at)
+    messages.incoming.created_since(agent_last_seen_at)
   end
 
   def push_event_data