refactor(verify-era-proof-attestation): replace watch channel with CancellationToken

Refactored stop signal handling in all components to use `tokio_util::sync::CancellationToken` instead of `tokio::sync::watch`.

- Improved cancellation logic by leveraging `CancellationToken` for cleaner and more efficient handling.
- Updated corresponding dependency to `tokio-util` version `0.7.14`.

bin/verify-era-proof-attestation/Cargo.toml

@@ -22,6 +22,7 @@ serde_yaml = "0.9.33"
 teepot.workspace = true
 thiserror.workspace = true
 tokio.workspace = true
+tokio-util = "0.7.14"
 tracing.workspace = true
 tracing-subscriber.workspace = true
 url.workspace = true