doc: releases: Bluetooth: Add note for GMAP BGS new requirement

Add note for the new requirement for the GMAP BGS role. There is
no GMAP sample or any changes in-tree that need to be updated for
this, so the best we can do, without implement a whole new sample,
is to refer to the BAP broadcast assistant sample.

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

Author: Thalley

doc/releases/migration-guide-4.3.rst

@@ -81,6 +81,9 @@ Bluetooth Audio
   :c:enumerator:`BT_AUDIO_CODEC_CFG_TARGET_PHY_2M`.
   The :c:macro:`BT_AUDIO_CODEC_CFG` macro defaults to these values.
   (:github:`93825``)
+* Setting the BGS role for GMAP now requires also supporting and implementing the
+  :kconfig:option:`CONFIG_BT_BAP_BROADCAST_ASSISTANT`.
+  See the :zephyr:code-sample:`bluetooth_bap_broadcast_assistant` sample as a reference.
 
 .. zephyr-keep-sorted-stop