Split Severity and Confidence values for vulnerability occurrences

What does this MR do?

severity and confidence both share the same list of values. This was fine and covering all our use cases until recently when trying to integrate Container Scanning and DAST results in the database.

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ee/issues/9397

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• EE specific content should be in the top level /ee folder
• For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?
• Security reports checked/validated by reviewer