move edition to 2021

Looks like since I've last looked at the repo, the update to
MSRV 1.56 happened, and we don't have any clippy errors on the 2021
edition, so I _think_ we can just up this value now.

Closes tokio-rs#553

Cargo.toml

@@ -12,7 +12,7 @@ readme = "README.md"
 description = "A Protocol Buffers implementation for the Rust Language."
 keywords = ["protobuf", "serialization"]
 categories = ["encoding"]
-edition = "2018"
+edition = "2021"
 
 [workspace]
 members = [