Switch to new ActiveRecord enum args, not deprecated in Rails 7.2

[jrochkind]

Rails 7.2 says:

DEPRECATION WARNING: Defining enums with keyword arguments is deprecated and will be removed
in Rails 8.0. Positional arguments should be used instead:

enum :kithe_model_type, {:collection=>0, :work=>1, :asset=>2}

Turns out new form is only available in Rails 7.0, and we're still supporting Rails 6.0 and 6.1 for some reason, so we'll put it in a condition on rails version.