Bump protobuf from 2.28.0 to 3.7.1 in /rust_grpcio_bench

Bumps [protobuf](https://github.com/stepancheg/rust-protobuf) from 2.28.0 to 3.7.1.
- [Changelog](https://github.com/stepancheg/rust-protobuf/blob/master/CHANGELOG-before-3.md)
- [Commits](stepancheg/rust-protobuf@v2.28.0...v3.7.1)

---
updated-dependencies:
- dependency-name: protobuf
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

rust_grpcio_bench/Cargo.lock

@@ -12,7 +12,7 @@ lto = true
 [dependencies]
 jemallocator = "0.5.0"
 grpcio = "0.12"
-protobuf = "2"
+protobuf = "3"
 futures = "0.3"
 
 [build-dependencies]