chore: drop `SEC_REGISTRY_IMAGE`

What does this MR do?

Drops hardcoded var in favor of project-level CI variable. This keeps the configuration centralized alongside username/password credentials for discoverability and allows us to copy-paste easily as we will be defining these using the new flattened structure as registry.gitlab.com/security-products/$CI_PROJECT_NAME

What are the relevant issue numbers?

Relates to https://gitlab.com/gitlab-org/gitlab/-/issues/334325

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