chore: Clarify primary_identifiers are unordered

What does this MR do?

Minor change to scan.primary_identifiers field to clarify that it's unordered.

What are the relevant issue numbers?

Fixes https://gitlab.com/gitlab-org/security-products/analyzers/report/-/issues/14

Checklist

• Ensure changes can be built upon without requiring a breaking change, see Building for Extensibility.
• Review and add/update tests for this feature/bug.
• Add an entry to the CHANGELOG if required, with the appropriate version. See Classifying Changes.
• Assign the MR to the appropriate person/people for review.