Remove `undefined` from Secure stage value for Severity and Confidence properties backend, frontend and documentation

Problem to solve

Per &2242 (comment 284527590)

• in %13.0 to drop support of undefined value for Severity and Confidence properties in the Security Report
• in %13.0 to drop support of undefined value for Severity and Confidence filters on the vulnerabilities API
• in %13.0 to remove the undefined value from our documentation

Intended users

Further details

Proposal

Permissions and Security

Documentation

Availability & Testing

There are currently specs for vulnerability management qa/specs/features/ee/browser_ui/secure/vulnerability_management_spec.rb and secure reporting qa/specs/features/ee/browser_ui/secure/security_reports_spec.rb

If no severity data is provided, it should be tested that 'Unknown' is shown rather than 'Undefined'. This should be added to these tests by the SET in addition to the lower level coverage which should be added by the engineer.

(Note that currently security_reports_spec is quarantined, as it relies on changes to the test tooling to facilitate testing feature flags)

What does success look like, and how can we measure that?

What is the type of buyer?

Links / references