Let's use the delegation to the transaction manager here

`within_new_transaction` is delegated to the transaction manager. Let's remove
this explicit receiver to honor the delegation and for consistency with usage
of the also delegated `current_transaction`.

activerecord/lib/active_record/connection_adapters/abstract/database_statements.rb

@@ -358,7 +358,7 @@ module ActiveRecord
           end
           yield current_transaction
         else
-          transaction_manager.within_new_transaction(isolation: isolation, joinable: joinable, &block)
+          within_new_transaction(isolation: isolation, joinable: joinable, &block)
         end
       rescue ActiveRecord::Rollback
         # rollbacks are silently swallowed