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`.

Cargo.lock

@@ -4962,9 +4962,9 @@ dependencies = [
 
 [[package]]
 name = "tokio-util"
-version = "0.7.13"
+version = "0.7.14"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "d7fcaa8d55a2bdd6b83ace262b016eca0d79ee02818c5c1bcdf0305114081078"
+checksum = "6b9590b93e6fcc1739458317cccd391ad3955e2bde8913edf6f95f9e65a8f034"
 dependencies = [
  "bytes",
  "futures-core",
@@ -5386,6 +5386,7 @@ dependencies = [
  "teepot",
  "thiserror 2.0.11",
  "tokio",
+ "tokio-util",
  "tracing",
  "tracing-subscriber",
  "url",