doc: releases: State zbus API as stable

Zbus has been around since Zephyr v3.3.0 and fulfills all the requirements
to be a stable API.

Signed-off-by: Rodrigo Peixoto <rodrigopex@gmail.com>

Author: rodrigopex

doc/releases/release-notes-4.2.rst

@@ -241,6 +241,7 @@ New APIs and options
 
 * ZBus
 
+  * Zbus has achieved stable status with the release of API version v1.0.0.
   * Runtime observers can work without heap. Now it is possible to choose between static, dynamic,
     and none allocation for the runtime observers nodes.
   * Runtime observers using :kconfig:option:`CONFIG_ZBUS_RUNTIME_OBSERVERS_NODE_ALLOC_NONE` must use