Update proc-macro2 requirement from =1.0.76 to =1.0.78

Updates the requirements on [proc-macro2](https://github.com/dtolnay/proc-macro2) to permit the latest version.
- [Release notes](https://github.com/dtolnay/proc-macro2/releases)
- [Commits](dtolnay/proc-macro2@1.0.76...1.0.78)

---
updated-dependencies:
- dependency-name: proc-macro2
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>

arrow-flight/gen/Cargo.toml

@@ -32,6 +32,6 @@ publish = false
 [dependencies]
 # Pin specific version of the tonic-build dependencies to avoid auto-generated
 # (and checked in) arrow.flight.protocol.rs from changing
-proc-macro2 = { version = "=1.0.76", default-features = false }
+proc-macro2 = { version = "=1.0.78", default-features = false }
 prost-build = { version = "=0.12.3", default-features = false }
 tonic-build = { version = "=0.10.2", default-features = false, features = ["transport", "prost"] }