Drop unused onboarding_progresses columns

What does this MR do and why?

This removes the following ignored columns from the onboarding_progresses table:

git_pull_at
subscription_created_at
scoped_label_created_at
security_scan_enabled_at
issue_auto_closed_at
repository_imported_at
repository_mirrored_at
secure_container_scanning_run_at
secure_secret_detection_run_at
secure_coverage_fuzzing_run_at
secure_cluster_image_scanning_run_at
secure_api_fuzzing_run_at

This is the next step following !160892 (merged):

which was included in %17.3.