Update common library to split Severity and Confidence sets of values for Vulnerabilities

We currently use the same levels for both our severity and confidence values in our analyzers. This works well for ~sast but not for ~"container scanning" or ~dast. We can initially map these values separately within GitLab via separate enums but should eventually handle this distinction upstream within the common library.

See related issue for handling the GitLab side of this work: https://gitlab.com/gitlab-org/gitlab-ee/issues/9397

Common Levels for both Severity and Confidence in common library: https://gitlab.com/gitlab-org/security-products/analyzers/common/blob/v2.1.3/issue/issue.go#L32-45