gomod: update grpc for GHSA-m425-mq94-257g (#736)
Ran "go get google.golang.org/grpc@v1.56.3". If I just use the latest
grpc it causes OTEL to update, which has a whole cascade of updates. I'd
rather tackle that in a seperate commit since last time I did that it
caused a bunch of pain in the sourcegraph repo.
Test Plan: go test