Update repo to `edition = "2021"` (#3900)

program/Cargo.toml

@@ -5,7 +5,7 @@ description = "Solana Program Library Token"
 authors = ["Solana Maintainers <maintainers@solana.foundation>"]
 repository = "https://github.com/solana-labs/solana-program-library"
 license = "Apache-2.0"
-edition = "2018"
+edition = "2021"
 exclude = ["js/**"]
 
 [features]