Increase default socket size for Opta

[ppp-one]

Hi - after 3 simultaneous http Ethernet connections to the Opta, it refuses further connections.

via python, the failed connections have:
('Connection aborted.', ConnectionResetError(54, 'Connection reset by peer'))

I notice "lwip.socket-max": 10 for mbed_app.json exists, but without building the source code it's not clear to me if this will fix the issue. I notice also MBED_CONF_LWIP_SOCKET_MAX in mbed_config.h exists, but changing this doesn't make a difference. I assume the libmbed.a needs building?

(for some context, I'm compiling/uploading my code to the Opta via the arduino-cli)

Thanks,
Peter