Update Vulnerabilities parsers after splitting of Severity and Confidence levels

We're splitting Severity and Confidence sets of values in https://gitlab.com/gitlab-org/gitlab-ee/issues/9397 and at the same time, we have string values of Severity and Confidence levels in DAST and Container Scanning parsers.

When this issue is done and DAST parser is merged, we have to:

  • consider making severity and confidence levels constants for DAST and Container Scanning parsers.
Edited by Tetiana Chupryna