Remove DAST-specific types and structs

What does this MR do?

• remove DAST category
• remove ZAProxy plugin identifier
• remove DAST-specific field from report
• remove DAST-specific fields from location
• bump format version to 2.4

What are the relevant issue numbers?

gitlab-org/gitlab#204783 (closed)

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; see discussion
• [-] Job definition updated, if necessary
• [-] Conforms to the code review guidelines
• [-] Conforms to the Go guidelines
• [-] Security reports checked/validated by reviewer