Update protobuf to 3.9.1

[elharo]

No description provided.

[BenWhitehead]

Doesn't this change require a corresponding bump to synthtool so that we have protoc 3.9.1 there?

[chingor13]

In theory, the generated code from an older version of protoc should be compatible with the newest protobuf version at runtime. The inverse is not the case.

I will run this through all the integration tests (they are skipped in the presubmit here)

[chingor13]

Spanner IT and Spanner JDBC IT tests failed.

Spanner test failed with "INVALID_ARGUMENT: com.google.api.gax.rpc.InvalidArgumentException: io.grpc.StatusRuntimeException: INVALID_ARGUMENT: projects/{project ID} is missing from uri."
and the JDBC IT test failed in ITTransactionRetryTest.testAbortWithConcurrentInsert. I rerunning to see if it was flakey

[chingor13]

gprc 1.23.0 has updated to 3.9