[Feature flag] Cleanup enforce_security_report_validation feature flag

Summary

This issue tracks the work required to cleanup the enforce_security_report_validation feature flag

See #351000 (closed) for the feature flag being enabled in production.

Checklist

• Create a merge request to remove enforce_security_report_validation feature flag. Ask for review and merge it.
  • Remove all references to the feature flag from the codebase.
  • Remove the YAML definitions for the feature from the repository.
  • Create a changelog entry.
• Ensure that the cleanup MR has been deployed to both production and canary. If the merge request was deployed before the code cutoff, the feature can be officially announced in a release blog post.
  • /chatops run auto_deploy status <merge-commit-of-cleanup-mr>
• Close the feature issue to indicate the feature will be released in the current milestone.
• Clean up the feature flag from all environments by running these chatops command in #production channel:
  • /chatops run feature delete enforce_security_report_validation --dev
  • /chatops run feature delete enforce_security_report_validation --staging
  • /chatops run feature delete enforce_security_report_validation
• Close this rollout issue.