GATT MTU size needs reconciled #22
Description
GATT MTU size is specified several places:
nrf/ble/nordic_ble_stack_observer.cc
nrf/ble/nordic_ble_stack.h
nrf/ble/att.h: static constexpr length_t const mtu_length_maximum = 251u
Get to the bottom of the actual GATT MTU maximum and provide one source of truth.