Change license metadata to SPDX 2.1

Cargo.toml

@@ -2,7 +2,7 @@
 name = "serde_bytes"
 version = "0.10.5" # remember to update html_root_url
 authors = ["David Tolnay <dtolnay@gmail.com>"]
-license = "MIT/Apache-2.0"
+license = "MIT OR Apache-2.0"
 description = "Optimized handling of `&[u8]` and `Vec<u8>` for Serde"
 homepage = "https://serde.rs"
 repository = "https://github.com/serde-rs/bytes"