chore: update Cargo.toml to change edition from 2026 to 2021

Author: Danialgharaie

Cargo.toml

@@ -13,7 +13,7 @@ resolver = "2"
 
 [workspace.package]
 version = "0.1.0"
-edition = "2026"
+edition = "2021"
 license = "MIT"
 authors = ["Danial Gharaie Amirabadi"]
 repository = "https://github.com/Danialgharaie/structscope"