Add more must_use attributes (#2846)

… so people get a warning when they accidentally add a semicolon after the response expression in a handler function.

Also update changelogs of axum-core, axum-extra.

axum-core/src/response/into_response.rs

@@ -111,6 +111,7 @@ use std::{
 /// ```
 pub trait IntoResponse {
     /// Create a response.
+    #[must_use]
     fn into_response(self) -> Response;
 }