Replace rubocop/rubocop-rspec with gitlab-styles

Replaces rubocop/rubocop-rspec with gitlab-styles after !65 (merged).

• Changes to load rubocop and rubocop-rspec via gitlab-styles 6.2.0 without no revision update on either Gem (as of this MR).
• Introduces cops from rubocop-gitlab-security, rubocop-performance, rubocop-rails Gems and their default configurations via gitlab-styles.
  • https://gitlab.com/gitlab-org/gitlab-styles/-/blob/v6.2.0/rubocop-default.yml
• Replaces only in-project RuboCop configurations duplicated with ones from gitlab-styles 6.2.0.
• Updates .rubocop_todo.yml by rubocop --auto-gen-config.
• Does not change any Ruby code (except for the gem DSL gitlab-labkit.gemspec)

Blocked by !67 (merged)

Closes #9 (closed)