Don't require ActiveRecord

[BjarneD]

The existing way to decide what method to look at as a condition to validation had 2 issues:

1. If a project weren't using ActiveRecord it couldn't use the method will_save_change_to_encrypted_password? However projects with a mongo database will be supporting this from version 8.1.0 https://jira.mongodb.com/browse/MONGOID-4528
2. If a project uses multiple databases. Eg. one using ActiveRecord and another using mongo before version 8.1.0. In this case ActiveRecord would be defined, however not on the model if the user model (resource) is a mongo based model. It would throw an error of undefined method.

The simple fix is to just check that the method exists.

Test passes with the fix:

devise-pwned_password % bin/test
Run options: --seed 898

# Running:

...........

Finished in 2.609481s, 4.2154 runs/s, 11.1133 assertions/s.
11 runs, 29 assertions, 0 failures, 0 errors, 0 skips

Thanks for creating and maintaining the gem.