Enable RSpec/EmptyLineAfterLetBlock cop

- enforce new line after let block rule.
- ignore existing offenses to be fixed in follow up:
  https://gitlab.com/gitlab-org/gitlab/-/issues/228860

FYI the existing offenses list was generated by running:

  bundle exec rubocop --auto-gen-config \
    --only "RSpec/EmptyLineAfterLetBlock" \
    --exclude-limit 1000