Verify DAST reports conforms to schema

What does this MR do?

• Each Test that produces a gl-dast-report.json report has the report checked against the schema
• Changes DAST output for severity and confidence to be title case, e.g. info to Info
• Adds field that is defined in the schema, remediations: []

What are the relevant issue numbers?

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated for GitLab EE, if necessary
• Documentation created/updated for this project, if necessary
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Job definition updated, if necessary
  • Job definition example
  • Vendored CI Templates (also in CE)
• Conforms to the code review guidelines
• Conforms to the Go guidelines
• Security reports checked/validated by reviewer