Update prost-build requirement from =0.12.4 to =0.12.6

Updates the requirements on [prost-build](https://github.com/tokio-rs/prost) to permit the latest version.
- [Release notes](https://github.com/tokio-rs/prost/releases)
- [Commits](tokio-rs/prost@v0.12.4...v0.12.6)

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

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

arrow-flight/gen/Cargo.toml

@@ -33,5 +33,5 @@ publish = false
 # Pin specific version of the tonic-build dependencies to avoid auto-generated
 # (and checked in) arrow.flight.protocol.rs from changing
 proc-macro2 = { version = "=1.0.82", default-features = false }
-prost-build = { version = "=0.12.4", default-features = false }
+prost-build = { version = "=0.12.6", default-features = false }
 tonic-build = { version = "=0.11.0", default-features = false, features = ["transport", "prost"] }