Deprecate `Rails::Generators::ActiveModel#update_attributes`

Deprecate #update_attributes in favor of `#update`.

ORMs that implement `Generators::ActiveModel#update_attributes` should change
to `#update`. Scaffold controller generators should change calls like:

    @orm_instance.update_attributes(...)

to:

    @orm_instance.update(...)

This goes along with the addition of `ActiveRecord::Base#update`,
introduced in cb2bd4aa619d9329c42aaf6d9f8eacc616ce53f4.