Docs: Add disabling cluster_image_scanning

Added a line in the documentation on how to disable cluster image
scanning when using Auto DevOps.

doc/topics/autodevops/customize.md

@@ -481,6 +481,7 @@ The following table lists variables used to disable jobs.
 | `brakeman-sast`                        | `SAST_DISABLED`                 |                       | If the variable is present, the job isn't created. |
 | `bundler-audit-dependency_scanning`    | `DEPENDENCY_SCANNING_DISABLED`  |                       | If the variable is present, the job isn't created. |
 | `canary`                               | `CANARY_ENABLED`                |                       | This manual job is created if the variable is present. |
+| `cluster_image_scanning`               | `CLUSTER_IMAGE_SCANNING_DISABLED` |                     | If the variable is present, the job isn't created. |
 | `code_intelligence`                    | `CODE_INTELLIGENCE_DISABLED`    | From GitLab 13.6      | If the variable is present, the job isn't created. |
 | `code_quality`                         | `CODE_QUALITY_DISABLED`         |                       | If the variable is present, the job isn't created. |
 | `container_scanning`                   | `CONTAINER_SCANNING_DISABLED`   |                       | If the variable is present, the job isn't created. |