Update prost-build requirement from =0.11.7 to =0.11.8 (#3767)

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](https://github.com/tokio-rs/prost/commits/v0.11.8)

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

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

arrow-flight/Cargo.toml

@@ -59,7 +59,7 @@ tower = "0.4.13"
 # Pin specific version of the tonic-build dependencies to avoid auto-generated
 # (and checked in) arrow.flight.protocol.rs from changing
 proc-macro2 = { version = "=1.0.51", default-features = false }
-prost-build = { version = "=0.11.7", default-features = false }
+prost-build = { version = "=0.11.8", default-features = false }
 tonic-build = { version = "=0.8.4", default-features = false, features = ["transport", "prost"] }
 
 [[example]]