Have the installed flow http server always listen on an ephemeral port.

Specifying a port of zero has the server listen on an ephemeral port.
Many users may not be aware of that unless they have a background in
networking where that's common practice. I'm also not able to think of
any use cases where listening on a hardcoded port would be beneficial,
so with this change I've opted to remove the ability entirely rather
than simply documenting that almost everybody should specify zero.

examples/test-installed/src/main.rs

@@ -19,7 +19,7 @@ fn main() {
         client.clone(),
         ad,
         secret,
-        yup_oauth2::InstalledFlowReturnMethod::HTTPRedirect(8081),
+        yup_oauth2::InstalledFlowReturnMethod::HTTPRedirect,
     );
     let mut auth = Authenticator::new_disk(
         client,