Update the edition of the solution format checker

Author: mo8it

Cargo.toml

@@ -15,7 +15,7 @@ authors = [
 ]
 repository = "https://github.com/rust-lang/rustlings"
 license = "MIT"
-edition = "2024" # On Update: Update the edition of the `rustfmt` command that checks the solutions.
+edition = "2024" # On Update: Update the edition of `rustfmt` in `dev check` and `CARGO_TOML` in `dev new`.
 rust-version = "1.85"
 
 [workspace.dependencies]

src/dev/check.rs

@@ -313,7 +313,7 @@ fn check_solutions(
     fmt_cmd
         .arg("--check")
         .arg("--edition")
-        .arg("2021")
+        .arg("2024")
         .arg("--color")
         .arg("always")
         .stdin(Stdio::null());