DEV: Fix migration that adds index to incoming_emails topic_id (#26007)

Why this change?

Follow up to f880f1a42f. When adding an
index concurrently where the database transaction is disabled, we have
to ensure that we drop the index first if it exists because an invalid
index can be created if the migration has failed before.

db/migrate/20240304030429_topic_id_on_incoming_email_index.rb

@@ -2,7 +2,12 @@
 class TopicIdOnIncomingEmailIndex < ActiveRecord::Migration[7.0]
   disable_ddl_transaction!
 
-  def change
+  def up
+    remove_index :incoming_emails, :topic_id, if_exists: true
     add_index :incoming_emails, :topic_id, algorithm: :concurrently
   end
+
+  def down
+    raise ActiveRecord::IrreversibleMigration
+  end
 end