Upgrade to latest grpc

[garrettjonesgoogle]

We currently depend on 1.10.1, but 1.12.0 is available now.

[yihanzhen]

@garrettjonesgoogle Can I just bump it manually or should this be done by codegen?

[garrettjonesgoogle]

Bump it manually, and run integration tests locally to make sure they pass (since CI doesn't run integration tests on PRs)