Update redis requirement from 0.20 to 0.21

Updates the requirements on [redis](https://github.com/mitsuhiko/redis-rs) to permit the latest version.
- [Release notes](https://github.com/mitsuhiko/redis-rs/releases)
- [Changelog](https://github.com/mitsuhiko/redis-rs/blob/master/CHANGELOG.md)
- [Commits](redis-rs/redis-rs@0.20.0...0.21.2)

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

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

Cargo.toml

@@ -33,7 +33,7 @@ orientdb-client = "0.6"
 postgres = "0.19"
 pretty_env_logger = "0.4"
 rdkafka = "0.26"
-redis = "0.20"
+redis = "0.21"
 reqwest = { version = "0.11", features = [ "blocking" ] }
 rusoto_core = "0.46"
 rusoto_credential = "0.46"