Upgrade etcd/grpc version

[strahe]

tidb current version: https://github.com/pingcap/tidb/blob/master/go.mod#L70

There may be some incompatibilities in the current version：

[omaskery]

We encountered this while trying to upgrade one of our projects to use the newest versions of the Go gRPC libraries, protoc versions, etc.

I had a brief look into creating a PR to address this but found quite quickly that this is currently all blocked on some stuff going on in the etcd Go client library: etcd-io/etcd#12398

Edit: the linked issue in etcd has been closed in favour of: etcd-io/etcd#12652

[disksing]

@omaskery Thank you for raising this issue, I have added it to the list and managed to put the preceding issue to the roadmap schedule of the PD project. Hopefully, we will upgrade it after the PD upgrade is completed before the release.