Replace SAST job definition with a vendored template

Problem to solve

Job definition for ~sast is frozen, and can't be easily updated without creating breaking changes.

Further details

With https://gitlab.com/gitlab-org/gitlab-ce/issues/53445, we'll be able to ship a template embedded with each version of GitLab. The template can be updated from one version to another, without impacting our users.

Proposal

The new official job definition is a single inclusion instruction:

include:
  template: SAST.gitlab-ci.yml

(see the discussion and final syntax)

Links / references

Execution

• Add the SAST.gitlab-ci.yml with the contents from the example to the templates dir under Security subdir
• Test in the development environment on a test project
• Update the ~Documentation for the SAST CI configuration
  • docs page, see https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/9875
  • security products release process (update a link in the section about vendored templates check)