Auto merge of #10442 - dtolnay-contrib:letunderscore, r=llogiq

Downgrade let_underscore_untyped to restriction

From reading #6842 I am not convinced of the cost/benefit of this lint even as a pedantic lint.

It sounds like the primary motivation was to catch cases of `fn() -> Result` being changed to `async fn() -> Result`. If the original Result was ignored by a `let _`, then the compiler wouldn't guide you to add `.await`. **However, this situation is caught in a more specific way by [let_underscore_future](https://rust-lang.github.io/rust-clippy/master/index.html#let_underscore_future) which was introduced _after_ the original suggestion (#9760).**

In #10410 it was mentioned twice that a <kbd>restriction</kbd> lint might be more appropriate for let_underscore_untyped.

changelog: Moved [`let_underscore_untyped`] to restriction

clippy_lints/src/let_underscore.rs

@@ -124,7 +124,7 @@ declare_clippy_lint! {
     /// ```
     #[clippy::version = "1.69.0"]
     pub LET_UNDERSCORE_UNTYPED,
-    pedantic,
+    restriction,
     "non-binding `let` without a type annotation"
 }