cargo: Bump rustc to 1.89 (#36818)

Update Rustc to 1.89.

Reviewable by commit.

Leftover work:
- #37330 
- #38777

---------

Signed-off-by: sagudev <16504129+sagudev@users.noreply.github.com>
Co-authored-by: sagudev <16504129+sagudev@users.noreply.github.com>

components/script/dom/transformstream.rs

@@ -1049,8 +1049,7 @@ impl TransformStreamMethods<crate::DomTypeHolder> for TransformStream {
                 }
             };
             let promise = if is_promise {
-                let promise = Promise::new_with_js_promise(result_object.handle(), cx);
-                promise
+                Promise::new_with_js_promise(result_object.handle(), cx)
             } else {
                 Promise::new_resolved(global, cx, result.get(), can_gc)
             };