Convert image to distroless

What does this MR do?

This MR updates the clangsa image to be based on UBI-Micro, which is currently considered (see Convert all GitLab Secure (security scanning) i... (gitlab-org/cloud-native/distroless&6 - closed) • Unassigned) the preferred distroless solution.

This MR also addresses the discussion in !8 (comment 2507403494) regarding using a non-root user.

What are the relevant issue numbers?

clangsa-sast: convert image to distroless (gitlab-org/gitlab#543080 - closed) • Hua Yan • 18.1

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
• Ensure the report version matches the equivalent schema version
• Conforms to the code review guidelines
• Conforms to the Go guidelines
• Security reports checked/validated by reviewer