Handle SIGTERM signals
Created by: eseliger
We want to handle SIGTERM in addition to SIGINT for graceful shutdowns. They are the mechanism used by k8s to inform about an impending pod termination. We currently don't support this and our applications seem to just exit immediately when they receive a SIGTERM. I don't really see what other harm this could cause, but I would love to capture k8s for an hour at some point to validate this is a good change. This should fix issues where gitserver is terminated during requests are currently being worked on (like git repo cloning or fetching) which we are seeing in server-side batch changes frequently.
Closes https://github.com/sourcegraph/sourcegraph/issues/27947