Fix warning on no edition set in serde_derive

New warning since nightly-2024-03-03:

    warning: serde_derive/Cargo.toml: no edition set: defaulting to the 2015 edition while 2018 is compatible with `rust-version`
    warning: serde_derive_internals/Cargo.toml: no edition set: defaulting to the 2015 edition while 2018 is compatible with `rust-version`

Author: dtolnay

serde_derive/Cargo.toml

@@ -5,6 +5,7 @@ authors = ["Erick Tryzelaar <[email protected]>", "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"
 homepage = "https://serde.rs"
 keywords = ["serde", "serialization", "no_std", "derive"]
 license = "MIT OR Apache-2.0"

serde_derive_internals/Cargo.toml

@@ -4,6 +4,7 @@ version = "0.29.0"
 authors = ["Erick Tryzelaar <[email protected]>", "David Tolnay <[email protected]>"]
 description = "AST representation used by Serde derive macros. Unstable."
 documentation = "https://docs.rs/serde_derive_internals"
+edition = "2015"
 exclude = ["build.rs"]
 homepage = "https://serde.rs"
 keywords = ["serde", "serialization"]