Add Vendor function to Identifier type

What does this MR do?

• Adds a new function Identifier.Vendor that returns the vendor that created the identifier. For example, CVE types will return NVD.
• Fix incorrect default report version. The report version emitted by default is 15.0.7 after the changes done in v4.2.0. This was unfortunately not done in the same MR.

What are the relevant issue numbers?

Update Gemnasium dependency scanner to emit a r... (gitlab-org/gitlab#422033 - closed)

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