Increase default socket size for Opta

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

Start by comparing the lwip.socket-max entry in mbed_app.json with MBED_CONF_LWIP_SOCKET_MAX in mbed_config.h, then trace how the Arduino CLI build uses libmbed.a. Reproduce the failure with more than three simultaneous HTTP Ethernet connections and verify that the resulting configuration allows them to remain connected.

Written by the indexing model from the issue text.