Raise exceptions when assigning to attr_readonly attributes

Since we don't use attr_readonly, this option doesn't really affect us.
So we're just using the new default value.

config/initializers/new_framework_defaults_7_1.rb

@@ -158,7 +158,7 @@ end
 # behavior would allow assignment but silently not persist changes to the
 # database.
 #++
-# Rails.application.config.active_record.raise_on_assign_to_attr_readonly = true
+Rails.application.config.active_record.raise_on_assign_to_attr_readonly = true
 
 ###
 # Enable validating only parent-related columns for presence when the parent is mandatory.