Update protobuf requirement

Updates the requirements on protobuf to permit the latest version.

Updates `protobuf` to 3.7.1

---
updated-dependencies:
- dependency-name: protobuf
  dependency-type: direct:production
  dependency-group: cargo
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

rust-to-elixir/rust-api-server/Cargo.toml

@@ -15,7 +15,7 @@ path = "src/lib.rs"
 grpcio = { version = "*", default-features = true, features = ["protobuf-codec","boringssl"] }
 grpcio-sys = { version = "*", default-features = true }
 libc = "*"
-protobuf = "^2"
+protobuf = "^3"
 futures = "*"
 futures-cpupool = "*"
 futures-channel = { version = "*", features = ["sink"] }