doc: releases: Add entry for bt_bap_unicast_client_unregister_cb

Add entry for new function
bt_bap_unicast_client_unregister_cb.

Signed-off-by: Emil Gydesen <emil.gydesen@nordicsemi.no>

Author: Thalley

doc/releases/release-notes-4.3.rst

@@ -135,6 +135,7 @@ New APIs and options
     * :c:struct:`bt_bap_stream` now contains an ``iso`` field as a reference to the ISO channel
     * :c:func:`bt_bap_unicast_group_get_info`
     * :c:func:`bt_cap_unicast_group_get_info`
+    * :c:func:`bt_bap_unicast_client_unregister_cb`
 
   * Host