refactor: add warning if severity not from vendor (or NVD or GH) is used

[DmitriyLewen]

Description

See https://github.com/aquasecurity/trivy/issues/6714#issuecomment-2116927641

Example:

Related issues

• Close #6714

Checklist

• [x] I've read the guidelines for contributing to this repository.
• [x] I've followed the conventions in the PR title.
• [ ] I've added tests that prove my fix is effective or that my feature works.
• [ ] I've updated the documentation with the relevant information (if needed).
• [ ] I've added usage information (if the PR introduces new options)
• [ ] I've included a "before" and "after" example to the description (if the PR is a user interface change).