Remove deprecated CS variables

What does this MR do and why?

Remove deprecated Container Scanning variables with Docker references Deprecation Announcement

All Container Scanning variables that are prefixed by DOCKER_ in variable name are deprecated. This includes the DOCKER_IMAGE, DOCKER_PASSWORD, DOCKER_USER, and DOCKERFILE_PATH variables. Support for these variables will be removed in the GitLab 16.0 release. Use the new variable names CS_IMAGE, CS_REGISTRY_PASSWORD, CS_REGISTRY_USER, and CS_DOCKERFILE_PATH in place of the deprecated names.

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Related to #2876 (closed)