insights: Modify OOB settings migration to be at 100% when it's a no-op

Created by: CristinaBirkel

See: https://github.com/sourcegraph/sourcegraph/issues/38555 And: https://github.com/sourcegraph/sourcegraph/pull/39010

This issue enables us to deprecate our oob settings migration.

When code insights is disabled, the oob settings migration always remains at 0%. This is a problem because when we go to deprecate it, admins will get a message saying that they need to contact Sourcegraph support because their migration didn't complete.

Off the top of my head, I think we could add a dummy row to the insights_settings_migration_jobs after adding all of the real rows, and then automatically mark it as complete. That way there will be something to signify that the migration is done even when there's no real work to do.