Log to stdout instead of stderr

I noticed that by default Rack::CommonLogger logs to env['rack.errors'], which default itself to $stderr: https://github.com/rack/rack/blob/5791ef617717d568dc3387cfd5db1c97f08455ca/lib/rack/handler/cgi.rb#L20

I believe this result in logs having the "error" level in K8s, e.g. https://console.cloud.google.com/kubernetes/deployment/us-central1/triage-serverless-prod/triage-serverless-12690061-production-2/triage-web/logs?folder=882286565318&project=gitlab-qa-resources&supportedpurview=project.

This MR changes the default to use $stdout so that messages from Rack::CommonLogger are classified as info messages (hopefully).