Fix failing to set app.deprecators in Rails <= 7.0

Refs. b25477d#r143749867

lib/carrierwave.rb

@@ -70,7 +70,7 @@ class Railtie < Rails::Railtie
       end
 
       initializer "carrierwave.deprecator" do |app|
-        app.deprecators[:carrierwave] = CarrierWave.deprecator
+        app.deprecators[:carrierwave] = CarrierWave.deprecator if app.respond_to?(:deprecators)
       end
 
       config.before_eager_load do