rabbit_db_vhost: Add `no_return()` to `update/2` spec

This function throws if the database fails to apply the transaction.
This function is only called by the `rabbit_vhost_limit` runtime
parameter module in its `notify/5` and `notify_clear/4` callbacks. These
callers have no way of handling this error but it should be very
difficult for them to face this crash: setting the runtime parameter
would need to succeed first which needs Khepri to be in majority. Khepri
would need to enter a minority between inserting/updating/deleting the
runtime parameter and updating the vhost. It's possible but unlikely.

In the future we could consider refactoring vhost limits to update the
vhost as the runtime parameter is changed, transactionally. I figure
that to be a very large change though so we leave this to the future.

deps/rabbit/src/rabbit_db_vhost.erl

@@ -349,7 +349,7 @@ list_in_khepri() ->
 -spec update(VHostName, UpdateFun) -> VHost when
       VHostName :: vhost:name(),
       UpdateFun :: fun((VHost) -> VHost),
-      VHost :: vhost:vhost().
+      VHost :: vhost:vhost() | no_return().
 %% @doc Updates an existing virtual host record using the result of
 %% `UpdateFun'.
 %%