Pet: Refactor instance method to use the Match model instead of the AdopterApplication model

[kasugaijin]

We have this instance method on Pet

def is_adopted?
adopter_applications.any? { |app| app.status == "adoption_made" }
end

The single source of truth on an adoption should be the Match model, where the match_type is adoption. Please update the method to query the pet's matches instead.