Bump RuboCop to 1.86.6

There were a few changes needed:

* Plugins are now specified through `plugin:` config keyword.
* All plugin gems need to be specified explicitly in Gemfile since they
  are no longer dependencies of plugins already specified explicitly.
* All plugin gems need to be updated in other to use the new APIs.
* One cop was renamed.
* New offenses safe to correct were corrected directly with `bundle exec
  rubocop -a`.
* New offenses unsafe to correct were added to the TODO configuration
  with `bundle exec rubocop --auto-gen-config --auto-gen-only-exclude
  --exclude-limit 1400 --no-auto-gen-timestamp`.

spec/validators/integer_array_validator_spec.rb

@@ -17,6 +17,7 @@ RSpec.describe IntegerArrayValidator do
     let(:instance) do
       Class.new do
         include ActiveModel::Validations
+
         attr_accessor :ids
 
         validates :ids, integer_array: true