Update GCP service account key pattern

What does this MR do?

Change GCP service account key detection pattern to look for a private_key value rather than the less-sensitive "type": "service_account" metadata. Preserve the service_account value as a keywords setting to avoid confusion with other types of keys.

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab/-/issues/375201+ (team members only)

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