style(rubocop): fix all safe auto correctable offenses

config/initializers/secret_key.rb

@@ -1,6 +1,6 @@
 if Postal.config.rails&.secret_key
   Rails.application.secrets.secret_key_base = Postal.config.rails.secret_key
 else
-  $stderr.puts "No secret key was specified in the Postal config file. Using one for just this session"
+  warn "No secret key was specified in the Postal config file. Using one for just this session"
   Rails.application.secrets.secret_key_base = SecureRandom.hex(128)
 end