Add explicit dast stage

In https://gitlab.com/gitlab-org/gitlab-exporter/-/merge_requests/143 my pipeline was completely broken until I did this. Thankfully, https://docs.gitlab.com/ee/user/application_security/dast/dast_troubleshooting.html#getting-error-dast-job-chosen-stage-does-not-exist-when-including-dast-ci-template says waht to do here - thanks @avielle for adding that in gitlab-org/gitlab!50539 (merged), it was very helpful!