Switch all crates to edition 2021

serde_derive/Cargo.toml

@@ -5,7 +5,7 @@ authors = ["Erick Tryzelaar <erick.tryzelaar@gmail.com>", "David Tolnay <dtolnay
 categories = ["no-std", "no-std::no-alloc"]
 description = "Macros 1.1 implementation of #[derive(Serialize, Deserialize)]"
 documentation = "https://serde.rs/derive.html"
-edition = "2015"
+edition = "2021"
 exclude = ["build.rs"]
 homepage = "https://serde.rs"
 keywords = ["serde", "serialization", "no_std", "derive"]