Use proc to set password length validator so it's possible to override it dynamically.

[nashby]

This change is useful for instances where applications have the need to support dynamic password lengths, without a system restart. Such applications will just have to override the password_length method in their own classes to support dynamic password lengths.

Gitlab uses it like this:

class User < ApplicationRecord
  ...
  class << self
    # Devise method overridden to allow support for dynamic password lengths
    def password_length
      Gitlab::CurrentSettings.minimum_password_length..Devise.password_length.max
    end
  end
end