This enables following cops:

Check for useless access modifiers

  Lint/UselessAccessModifier

Checks for attempts to use `private` or `protected` to set the
visibility of a class method, which does not work.

  Lint/IneffectiveAccessModifier

This also disables two false possitives in concerns.

See:

https://github.com/rails/rails/commit/af53280a4b5b3323ac87dc60deb2b1b781197b2b

We assume that when someone adds a key for the configuration entry, but
does not provide a valid value, which causes entry to be `nil`, then
entry should be considered as the undefined one. We also assume this is
semantically correct, this is also backwards compatible with legacy CI
config processor.

See issue #18775 for more details.

This makes use of `ActiveModel::Validations` encapsulated in a separate
class that is accessible from a node object.

This reverts commit 13e37a3e.