Update prost-build requirement from =0.12.1 to =0.12.2 (#8244)

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@v0.12.1...v0.12.2)

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

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

datafusion/proto/gen/Cargo.toml

@@ -32,4 +32,4 @@ publish = false
 [dependencies]
 # Pin these dependencies so that the generated output is deterministic
 pbjson-build = "=0.6.2"
-prost-build = "=0.12.1"
+prost-build = "=0.12.2"