Update prost requirement from 0.11 to 0.12

Updates the requirements on [prost](https://github.com/tokio-rs/prost) to permit the latest version.
- [Release notes](https://github.com/tokio-rs/prost/releases)
- [Commits](tokio-rs/prost@prost-build-0.11.1...v0.11.9)

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

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

Cargo.toml

@@ -21,7 +21,7 @@ env_logger = "0.10"
 futures = "0.3"
 log = "^0.4"
 paho-mqtt = "0.12"
-prost = "0.11"
+prost = "0.12"
 prost-types = "0.11"
 serde = "1.0.160"
 serde_derive = "1.0.163"