upgrading: note about Service is not from tower

_stable/upgrading.md

@@ -63,6 +63,12 @@ use the [`server::conn::auto::Builder`][auto] from [`hyper-util`][].
 
 The listening server acceptor can be replaced with a simple loop.
 
+## `Service`/`service_fn`
+
+Previously, hyper depended on `tower` for the `Service` trait. Because
+`tower` is not yet 1.0, hyper could not publicly depend on it. So, it's
+`Service` trait and the helper function `service_fn` are defined in
+`hyper::service`.
 
 [changelog]: https://github.com/hyperium/hyper/blob/master/CHANGELOG.md#v100-2023-11-15
 [`hyper-util`]: https://crates.io/crates/hyper-util