[nrf fromlist] samples: net: wifi: Use fixed data buffer size for nuc…

…leo_h723zg target

Enable fixed data buffers for nucleo_h723zg target to prevent RAM overflows
with variable network buffers usage.

Upstream PR: zephyrproject-rtos/zephyr#78852

Signed-off-by: Triveni Danda <triveni.danda@nordicsemi.no>

samples/net/wifi/boards/nucleo_h723zg.conf

@@ -0,0 +1 @@
+CONFIG_NRF_WIFI_NET_BUF_FIXED_SIZE=y