Dangerfiles are located by default under the danger/**/*. So this change
moves dangerfiles there to simplify configuration.

By calling import_dangerfiles instead of importing single files
separately, we'll also inherit all the Dangerbot rules. Including the
reviewer roulette: https://gitlab.com/gitlab-org/ruby/gems/gitlab-dangerfiles#rules

Closes https://gitlab.com/gitlab-org/distribution/team-tasks/-/issues/1417