Drop namespace_settings columns related to security policy pipeline execution action experiment

Why are we doing this work

With !159607 (merged), we ignored columns in NamespaceSetting:

• toggle_security_policy_custom_ci
• lock_toggle_security_policy_custom_ci

In %17.4, we should drop these columns.

Relevant links

• Processing removed in !159607 (merged)

Implementation plan

• Create a migration to drop the columns

milestone '17.4'

include Gitlab::Database::MigrationHelpers::CascadingNamespaceSettings

enable_lock_retries!

def up
  remove_cascading_namespace_setting :toggle_security_policy_custom_ci
end

def down
  add_cascading_namespace_setting :toggle_security_policy_custom_ci, :boolean, default: false, null: false
end

Verification steps