Move to SAST_EXCLUDED_ANALYZERS

What does this MR do?

As part of gitlab-org/gitlab#290777 (closed), move away from SAST_DEFAULT_ANALYZERS and utilize SAST_EXCLUDED_ANALYZERS.

Also move the definition(s) for SAST_DEFAULT_ANALYZERS and SAST_EXCLUDED_ANALYZERS to .qa-downstream-sast: definition to reduce duplication.

What are the relevant issue numbers?

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
  • Auto-DevOps template
  • Job definition example
  • CI Templates
• Conforms to the code review guidelines
• Conforms to the Go guidelines
• Security reports checked/validated by reviewer