Turn aggregated vulnerabilities ON by default

What does this MR do?

Do not merge until the URL list portion of gitlab-org/gitlab#285144 (closed) is complete. This is being done in gitlab-org/gitlab!58206 (merged) and gitlab-org/gitlab!61851 (merged) and shouldn't be merged until it's in production.

This MR removes the --ff-aggregated-vulnerabilities feature flag and adds --aggregated-vulnerabilities and DAST_AGGREGATE_VULNERABILITIES, both of which are True by default.

These changes are documented by gitlab-org/gitlab!59195 (merged)

What are the relevant issue numbers?

gitlab-org/gitlab#254043 (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
• 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