Deprecate and remove SAST_ANALYZER_IMAGE_PREFIX and DS_ANALYZER_IMAGE_PREFIX in the Security templates

Follow up of !28617 (comment 319759132) We can't remove a var without deprecating it first. This issue is to make sure we don't forget to deprecate the var, and clean-up https://gitlab.com/gitlab-org/gitlab/-/blob/master/lib/gitlab/ci/templates/Security/SAST.gitlab-ci.yml once removed (replace with SECURE_ANALYZERS_PREFIX).

Related Links

Deprecation release post: gitlab-com/www-gitlab-com!49202 (merged)

Implementation Plan

• Update SAST vendored template to no longer use SAST_ANALYZER_IMAGE_PREFIX
• Update Dependency Scanning vendored template to no longer use DS_ANALYZER_IMAGE_PREFIX
• Update SAST documentation to not reference deprecated variable.
• Update Dependency Scanning vendored template to no longer use deprecated variable.