Bluetooth: host: Rewrite `bt_gatt_notify_multiple`

This API now becomes a low-level API, in the sense that it should only be
used if the app developer knows what he is doing and wants to ensure a
ATT_MULTIPLE_HANDLE_VALUE_NTF PDU goes on-air.

For the other 99% of use cases, `bt_gatt_notify` should instead be used, as
it will automatically upgrade to ATT_MULTIPLE_HANDLE_VALUE_NTF when
possible.

One can disable the batching of notifications when using `bt_gatt_notify`
by setting CONFIG_BT_GATT_NOTIFY_MULTIPLE_FLUSH_MS=0 .

This API doesn't support lookup by UUID any more.

Signed-off-by: Jonathan Rico <jonathan.rico@nordicsemi.no>