Update SD Ruleset version to 0.20.0

What does this MR do?

This MR updates SD Rules version to v0.20.0 to support new rules for SPP:

• Added support for 21 new credential types (including AWS Cloud Provider)
• Enhanced rule quality and coverage for 6 existing rules
• Removed rules: AWSSTSKey, Tailscale key, and Grafana API token

Read more details about the rule changes in the linked issue.

What are the relevant issue numbers?

Bump secret-detection-rules to version v0.20.0 ... (gitlab-org/gitlab#579077) • Vishwa Bhat • 18.6

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated for this project, if necessary
• Documentation reviewed by technical writer or follow-up review issue created
• Tests updated/added for this feature/bug
• Job definition updated, if necessary
  • Auto-DevOps template
  • Job definition example
  • CI Templates
• Conforms to the code review guidelines