Add a DAST.latest CI template

This template does not include stages, which might be a breaking change
for users who do not already have a `dast` stage defined.

changelogs/unreleased/254325-remove-stages-from-dast-template.yml

+---
+title: Add DAST.latest.gitlab-ci.yml
+merge_request: 50539
+author:
+type: added

doc/user/application_security/dast/index.md

@@ -86,6 +86,20 @@ variables:
   DAST_WEBSITE: https://example.com
 ```
 
+### Latest template
+
+> [Introduced](https://gitlab.com/gitlab-org/gitlab/-/issues/254325) in GitLab 13.8
+
+To use the latest version of the DAST template, include
+`DAST.latest.gitlab-ci.yml` instead of `DAST.gitlab-ci.yml`.
+See the CI [docs](../../../development/cicd/templates.md#latest-version)
+on template versioning for more information.
+
+Please note that the latest version may include breaking changes. Check the
+[DAST troubleshooting guide](#troubleshooting) if you experience problems.
+
+### Template options
+
 There are two ways to define the URL to be scanned by DAST:
 
 1. Set the `DAST_WEBSITE` [variable](../../../ci/yaml/README.md#variables).
@@ -1040,6 +1054,25 @@ If your DAST job exceeds the job timeout and you need to reduce the scan duratio
 
 For information on this, see the [general Application Security troubleshooting section](../../../ci/pipelines/job_artifacts.md#error-message-no-files-to-upload).
 
+### Getting error `dast job: chosen stage does not exist` when including DAST CI template
+
+Newer versions of the DAST CI template do not define stages in order to avoid
+overwriting stages from other CI files. If you've recently started using
+`DAST.latest.gitlab-ci.yml` or upgraded to a new major release of GitLab and
+began receiving this error, you will need to define a `dast` stage with your
+other stages. Please note that you must have a running application for DAST to
+scan. If your application is set up in your pipeline, it must be deployed
+ in a stage _before_ the `dast` stage:
+
+```yaml
+stages:
+  - deploy  # DAST needs a running application to scan
+  - dast
+
+include:
+  - template: DAST.latest.gitlab-ci.yml
+```
+
 <!-- ## Troubleshooting
 
 Include any troubleshooting steps that you can foresee. If you know beforehand what issues

lib/gitlab/ci/templates/Security/DAST.latest.gitlab-ci.yml

+# Read more about this feature here: https://docs.gitlab.com/ee/user/application_security/dast/
+
+# Configure the scanning tool through the environment variables.
+# List of the variables: https://docs.gitlab.com/ee/user/application_security/dast/#available-variables
+# How to set: https://docs.gitlab.com/ee/ci/yaml/#variables
+
+variables:
+  DAST_VERSION: 1
+  # Setting this variable will affect all Security templates
+  # (SAST, Dependency Scanning, ...)
+  SECURE_ANALYZERS_PREFIX: "registry.gitlab.com/gitlab-org/security-products/analyzers"
+
+dast:
+  stage: dast
+  image:
+    name: "$SECURE_ANALYZERS_PREFIX/dast:$DAST_VERSION"
+  variables:
+    GIT_STRATEGY: none
+  allow_failure: true
+  script:
+    - export DAST_WEBSITE=${DAST_WEBSITE:-$(cat environment_url.txt)}
+    - if [ -z "$DAST_WEBSITE$DAST_API_SPECIFICATION" ]; then echo "Either DAST_WEBSITE or DAST_API_SPECIFICATION must be set. See https://docs.gitlab.com/ee/user/application_security/dast/#configuration for more details." && exit 1; fi
+    - /analyze
+  artifacts:
+    reports:
+      dast: gl-dast-report.json
+  rules:
+    - if: $DAST_DISABLED
+      when: never
+    - if: $DAST_DISABLED_FOR_DEFAULT_BRANCH &&
+          $CI_DEFAULT_BRANCH == $CI_COMMIT_REF_NAME
+      when: never
+    - if: $CI_DEFAULT_BRANCH != $CI_COMMIT_REF_NAME &&
+          $REVIEW_DISABLED && $DAST_WEBSITE == null &&
+          $DAST_API_SPECIFICATION == null
+      when: never
+    - if: $CI_COMMIT_BRANCH &&
+          $CI_KUBERNETES_ACTIVE &&
+          $GITLAB_FEATURES =~ /\bdast\b/
+    - if: $CI_COMMIT_BRANCH &&
+          $GITLAB_FEATURES =~ /\bdast\b/ &&
+          $DAST_WEBSITE
+    - if: $CI_COMMIT_BRANCH &&
+          $GITLAB_FEATURES =~ /\bdast\b/ &&
+          $DAST_API_SPECIFICATION