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]>

datafusion/proto/gen/Cargo.toml

@@ -35,4 +35,4 @@ workspace = true
 [dependencies]
 # Pin these dependencies so that the generated output is deterministic
 pbjson-build = "=0.6.2"
-prost-build = "=0.12.4"
+prost-build = "=0.12.6"