Transition to rules syntax for SAST vendored template

As a part of #36539 (closed), we need to transition to rules: syntax as only/except is considered deprecated

https://gitlab.com/gitlab-org/gitlab/blob/master/lib/gitlab/ci/templates/Security/SAST.gitlab-ci.yml

Along with this work we will need to update the docs around detection logic that was introduced with #211694 (closed)