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This changes from the previous limit of 15 characters.  Although DHCP and
mDNS allow for up to 63, ESP32 and ESP8266 only allow 32, so this seems
like a reasonable limit to enforce across all ports (and avoids wasting the
additional memory).

Also clarifies that `MICROPY_PY_NETWORK_HOSTNAME_MAX_LEN` does not include
the null terminator (which was unclear before).

This work was funded through GitHub Sponsors.
Signed-off-by: Jim Mussared <jim.mussared@gmail.com>