chore: upgrade to rails 7.1 and ruby 3.4 (#3457)

config/initializers/secret_key.rb

@@ -1,8 +1,8 @@
 # frozen_string_literal: true
 
 if Postal::Config.rails.secret_key
-  Rails.application.secrets.secret_key_base = Postal::Config.rails.secret_key
+  Rails.application.credentials.secret_key_base = Postal::Config.rails.secret_key
 else
   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)
+  Rails.application.credentials.secret_key_base = SecureRandom.hex(128)
 end