Update lint-docs to default to Rust 2024

This updates the lint-docs tool to default to the 2024 edition. The lint
docs are supposed to illustrate the code with the latest edition, and I
just forgot to update this in
https://github.com/rust-lang/rust/pull/133349.

Some docs needed to add the `edition` attribute since they were assuming
a particular edition, but were missing the explicit annotation.

compiler/rustc_lint/src/builtin.rs

@@ -948,7 +948,7 @@ declare_lint! {
     ///
     /// ### Example
     ///
-    /// ```rust,compile_fail
+    /// ```rust,compile_fail,edition2021
     /// #[no_mangle]
     /// const FOO: i32 = 5;
     /// ```