Add 12.8 job definition to .gitlab-ci.yml

What does this MR do?

Adds 12.8 job definition for SAST.

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/security-products/release/issues/93

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated for GitLab EE, if necessary
• Documentation created/updated for this project, if necessary
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Job definition updated, if necessary
  • Job definition example
  • CI Templates (to be removed)
  • Vendored CI Templates (also in CE)
  • Ensure that all of the ENV vars supported are propagated to Docker containers upon docker run in job definitions
• Conforms to the code review guidelines
• Conforms to the Go guidelines
• Security reports checked/validated by reviewer