samples: lz4: reduce malloc arena size

PR #63332 introduced a change to the LZ4 sample that set the malloc
arena to an unnecessarily large size.
After testing on native_posix, qemu_m3, and esp32s3_devkitm_appcpu, it
would appear 24K is a much more reasonable (and sufficient) size.

Signed-off-by: Benjamin Cabé <benjamin@zephyrproject.org>