[EE] Upgrade Rubocop to 0.68.1

This adds Ruby 2.6 support.

The bulk of changes were generated with three commands:

1. bundle exec rubocop --auto-gen-config
2. bundle exec rubocop --a
3. bundle exec haml-lint --auto-gen-config

CE port: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/28046