Update Rust crate futures-util to v0.3.21

Author: renovate-bot

Cargo.lock

@@ -56,7 +56,7 @@ diesel_migrations = { version = "=1.4.0", features = ["postgres"] }
 dotenv = "=0.15.0"
 flate2 = "=1.0.22"
 futures-channel = { version = "=0.3.21", default-features = false }
-futures-util = "=0.3.19"
+futures-util = "=0.3.21"
 hex = "=0.4.3"
 http = "=0.2.6"
 hyper = { version = "=0.14.16", features = ["client", "http1"] }