samples: Bluetooth: Reduce heap for hci_ipc

Reducing the heap requirement for the hci_ipc sample from
8KiB to 4KiB. It has been tested in various configurations to
work with 2300, but 4KiB was used to provide some leeway for
future updates.

The heap requirement is not documented, and it is unclear why
values lower than 2300 (not exact number) won't boot, and should
be investigated further.

Memory has become a very scarce resource for especially the ISO
builds for the nRF5340, which is why this is getting reduced.

Signed-off-by: Emil Gydesen <emil.gydesen@nordicsemi.no>