Update GRPC to latest release

[roidelapluie]

@beorn7 @codesome

In the latest release, we had to rollback the GRPC release.

Can we now update GRPC for v1.31.0? (see #7621)

It is annoying not to update it because in the last month some dependencies depend on newer releases.

[roidelapluie]

etcd-io/etcd#12124 is still not fixed :(

[codesome]

And looks like it is going to take some time etcd-io/etcd#12124 (comment), but I see some push there. Upgrading gRPC in Prometheus would entirely block Cortex from vendoring Prometheus :/

If the dependencies in Prometheus which depend on latest gRPC are not critical to be upgraded or not blocking any work, we could consider waiting for it to be upgraded in etcd.

[zmt]

This comment in go.mod led me to this issue, but this issue can almost certainly be closed and that comment removed since I see many newer versions of grpc in go.sum