Procfile: increase gunicorn timeout to 5 minutes

Pulling all of the information from the OpenStack API that is needed to populate the statistics table on the app's landing page take quite a while. Sometimes 60 seconds is long enough, but seemingly only if there have been enough recent attempts that have warmed up OpenStack's answers. This slowness is why the app tries to cache the stats for 25 hours and also has a cron job to refresh that cache once per day.

When we were using uwsgi to run the app folks seemed to notice the landing page 502's a lot less. We did not have any explicit timeouts in the uwsgi configuration. Let's try a 5 minute (300 second) with gunicorn to see if that helps.

Bug: T337265