Depend on ruby/debug, replacing Byebug

ruby/debug is a new debugger that is going to ship with CRuby.

It makes sense for Rails to switch to this one because that is
where the language is heading, and because Byebug is not fully
compatible with Zeitwerk. See

    deivid-rodriguez/byebug#564

While ruby/debug has not been heavily tested with Zeitwerk,
casual usage seems to suggest it works without issues, including
explicit namespaces, which is where Byebug and Zeitwerk conflict.

Byebug is terrific, thanks a lot for all these years. ❤️

activestorage/test/test_helper.rb

@@ -12,11 +12,6 @@
 require "active_storage/service/mirror_service"
 require "image_processing/mini_magick"
 
-begin
-  require "byebug"
-rescue LoadError
-end
-
 require "active_job"
 ActiveJob::Base.queue_adapter = :test
 ActiveJob::Base.logger = ActiveSupport::Logger.new(nil)