Revert "Samples: Philosophers: Revert stack size change"

This reverts commit 7c00ecbbbfb75bd87c595de65625d319e416be1c.

Changes introduced in that commit break the Dining Philosophers demo
for the following in-tree platforms:
  - actinius_icarus
  - circuitdojo_feather_nrf9160
  - nrf5340_audio_dk_nrf5340_cpuapp
  - nrf5340dk_nrf5340_cpuapp

Fixes https://github.com/zephyrproject-rtos/zephyr/issues/55040.

Signed-off-by: Saku Rautio <saku.rautio@nordicsemi.no>