Bump protobuf from 2.28.0 to 3.7.2 in /testbench/rust

Bumps [protobuf](https://github.com/stepancheg/rust-protobuf) from 2.28.0 to 3.7.2.
- [Changelog](https://github.com/stepancheg/rust-protobuf/blob/master/CHANGELOG-before-3.md)
- [Commits](stepancheg/rust-protobuf@v2.28.0...v3.7.2)

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

Author: BjoernAtBosch

testbench/rust/Cargo.lock

@@ -6,5 +6,5 @@ edition = "2021"
 # See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
 
 [dependencies]
-protobuf        = "~2"
+protobuf        = "~3"
 futures         = "~0.3"