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>