Increase network buffer sizes to fix buffer allocation failures

[cdwilson]

I haven't extensively tested this, but this looks like it fixes the errors like the following:

[00:00:09.514,617] <err> net_buf: pkt_alloc_buffer():876: Failed to get free buffer
[00:00:09.514,648] <err> net_pkt: Data buffer (704) allocation failed.
[00:00:09.614,044] <err> net_buf: pkt_alloc_buffer():876: Failed to get free buffer
[00:00:09.614,074] <err> net_pkt: Data buffer (670) allocation failed.
[00:00:09.614,074] <wrn> net_conn: pkt cloning failed, pkt 0x20038c58 dropped
[00:00:10.737,854] <err> net_buf: pkt_alloc_buffer():876: Failed to get free buffer
[00:00:10.737,884] <err> net_pkt: Data buffer (742) allocation failed.
[00:00:10.737,884] <wrn> net_conn: pkt cloning failed, pkt 0x20038c58 dropped

For more info, see https://developer.nordicsemi.com/nRF_Connect_SDK/doc/latest/nrf/working_with_nrf/nrf70/developing/constrained.html#networking-stack