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...v0.12.1)

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

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

actix-protobuf/Cargo.toml

@@ -21,8 +21,8 @@ all-features = true
 actix-web = { version = "4", default-features = false }
 derive_more = "0.99.7"
 futures-util = { version = "0.3.7", default-features = false }
-prost = { version = "0.11", default-features = false }
+prost = { version = "0.12", default-features = false }
 
 [dev-dependencies]
 actix-web = { version = "4", default-features = false, features = ["macros"] }
-prost = { version = "0.11", default-features = false, features = ["prost-derive"] }
+prost = { version = "0.12", default-features = false, features = ["prost-derive"] }