Update tokio requirement from ~1.14 to ~1.21 in /lightning-block-sync

Updates the requirements on [tokio](https://github.com/tokio-rs/tokio) to permit the latest version.
- [Release notes](https://github.com/tokio-rs/tokio/releases)
- [Commits](tokio-rs/tokio@tokio-1.14.0...tokio-1.21.0)

---
updated-dependencies:
- dependency-name: tokio
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

lightning-block-sync/Cargo.toml

@@ -27,4 +27,4 @@ serde_json = { version = "1.0", optional = true }
 chunked_transfer = { version = "1.4", optional = true }
 
 [dev-dependencies]
-tokio = { version = "~1.14", features = [ "macros", "rt" ] }
+tokio = { version = "~1.21", features = [ "macros", "rt" ] }