[zephyr] Drop BLE connection on endpoint close (#14600)

When a BLE endpoint is closed on the peripheral side, the
associated connection is not automatically closed. Instead,
BlePlatformDelegate::NotifyChipConnectionClosed() is called,
but the function is empty for Zephyr and many other
platforms.

Implement connection closing for Zephyr platform.

Signed-off-by: Damian Krolik <damian.krolik@nordicsemi.no>

src/platform/Zephyr/BLEManagerImpl.cpp

@@ -767,7 +767,7 @@ bool BLEManagerImpl::SendReadResponse(BLE_CONNECTION_OBJECT conId, BLE_READ_REQU
 
 void BLEManagerImpl::NotifyChipConnectionClosed(BLE_CONNECTION_OBJECT conId)
 {
-    // Intentionally empty.
+    CloseConnection(conId);
 }
 
 bool BLEManagerImpl::IsSubscribed(bt_conn * conn)