Update protobuf requirement from 2.0 to 3.1

Updates the requirements on [protobuf](https://github.com/stepancheg/rust-protobuf) to permit the latest version.
- [Release notes](https://github.com/stepancheg/rust-protobuf/releases)
- [Changelog](https://github.com/stepancheg/rust-protobuf/blob/master/CHANGELOG.md)
- [Commits](stepancheg/rust-protobuf@v2.0.0...v3.1.0)

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

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

Author: dependabot[bot]

Cargo.toml

@@ -36,7 +36,7 @@ smallvec = "1.7"
 inferno = { version = "0.11", default-features = false, features = ["nameattr"], optional = true }
 prost = { version = "0.10", optional = true }
 prost-derive = { version = "0.10", optional = true }
-protobuf = { version = "2.0", optional = true }
+protobuf = { version = "3.1", optional = true }
 criterion = {version = "0.3", optional = true}
 
 [dependencies.symbolic-demangle]