Add specs for legacy DAST report

We still support the legacy format, but from !17778 (merged), the legacy format is no longer used in tests.

We need to update dast_spec.rb (similar to the table-based tests present in dependency_scanning.rb)

Legacy Report: gl-dast-report-legacy.json

From #13034 (closed):

Move outdated formats to https://gitlab.com/gitlab-org/gitlab-ee/tree/master/spec/fixtures/security-reports/deprecated and keep under test