ci: upgrade protobuf to v3.11.3 (googleapis/google-cloud-cpp-spanner#1247)
ci: upgrade protobuf to v3.11.3 (googleapis/google-cloud-cpp-spanner#1247)

Upgrade protobuf to 3.11.3, this is needed to build with the latest and
greatest googleapis on Windows.
googleapis/google-cloud-cpp,googleapis/google-cloud-cpp,googleapis/google-cloud-cpp,googleapis/google-cloud-cpp