chore: update to 2021 edition

Author: seanmonstar

Cargo.toml

@@ -10,7 +10,7 @@ license = "MIT"
 authors = ["Sean McArthur <[email protected]>"]
 keywords = ["http", "hyper", "hyperium"]
 categories = ["network-programming", "web-programming::http-client", "web-programming::http-server"]
-edition = "2018"
+edition = "2021"
 
 [package.metadata.docs.rs]
 features = ["full"]