Use Rails method to get RabbitMQ config

The previous implementation isn't compatible with Ruby 3.1 due to the
upgrade of Psych (YAML library) to 4.0 [1] in stdlib and the change in `load_file`.

A simpler way to replace this is to use the Rails built in method to
load a config file.

[1]: ruby/psych#488

lib/tasks/setup_exchange.rake

@@ -1,6 +1,6 @@
 desc "Create RabbitMQ exchanges"
 task setup_exchange: :environment do
-  config = YAML.load_file(Rails.root.join("config/rabbitmq.yml"))[Rails.env].symbolize_keys
+  config = Rails.application.config_for(:rabbitmq)
 
   bunny = Bunny.new(ENV["RABBITMQ_URL"])
   channel = bunny.start.create_channel