doc: releases: add migration note for bt_conn_br_cb refactor

Add a migration guide entry for the BR/EDR callback restructuring
introduced in PR zephyrproject-rtos#108022. The role_changed and br_mode_changed
callbacks in bt_conn_cb have been moved into a new bt_conn_br_cb
sub-struct, requiring users to update designated initializers.

Signed-off-by: Kai Cheng <chengkai@xiaomi.com>

Author: chengkai15

doc/releases/migration-guide-4.5.rst

@@ -176,6 +176,19 @@ Bluetooth Audio
 
 .. zephyr-keep-sorted-stop
 
+Bluetooth Classic
+=================
+
+* The BR/EDR specific callbacks ``role_changed`` and ``br_mode_changed`` in
+  :c:struct:`bt_conn_cb` have been moved into a new sub-struct
+  :c:struct:`bt_conn_br_cb`, accessible via the ``br`` member. Application code
+  using these callbacks must update the designated initializers:
+
+  * ``.role_changed`` → ``.br.role_changed``
+  * ``.br_mode_changed`` → ``.br.mode_changed``
+
+  (:github:`108022`)
+
 Bluetooth HCI
 =============