databases/rubygem-activemodel81: Add rubygem-activemodel81 8.1.0

Active Model provides a known set of interfaces for usage in model classes. They
allow for Action Pack helpers to interact with non-Active Record models, for
example. Active Model also helps with building custom ORMs for use outside of
the Rails framework.

Active Model provides a default module that implements the basic API required to
integrate with Action Pack out of the box: ActiveModel::API.