misc: Update Rust edition

Author: Luni-4

Cargo.toml

@@ -6,7 +6,7 @@ description = "A Rust JSON structural diff library"
 repository = "https://github.com/Luni-4/json-structural-diff"
 keywords = ["json-structural-diff", "json-diff"]
 license = "MIT"
-edition = "2018"
+edition = "2021"
 
 [features]
 colorize = ["console"]