Remove support for parsing legacy DAST JSON reports

Proposal

DAST released version the MAJOR version 2.0.0 when GitLab 14.0 was released which removed support for the old ZAP JSON report format.

This issue proposes that the code to parse the legacy fields be removed from the GitLab Rails codebase.

Evidence

Version 2.3 is the first valid Secure schema version and was introduced in March 2020.

Risks

DAST started using the Secure report format fields in December 2019 at version v1.6.0. Should this issue be played then DAST scans prior to this date/version will not integrate successfully into the Security Dashboard.

19.1% of DAST scans use version 2.0. This isn't a valid Secure schema version, so these likely represent scans run with the legacy ZAP JSON report format. This change will likely break those scans ability to integrate into the Security Dashboard.