Added some missing baudrates

[cmaglie]

This is a tentative fix to add missing baud rates. It is known that there may be some issues on some OS, so testing is required to see if everything is working as expected.

The Arduino IDE 1.8.x has the following baud rates in the selector:

protected String[] serialRateStrings = {
  "300", "1200", "2400", "4800", "9600",
  "19200", "38400", "57600", "74880",
  "115200", "230400", "250000", "500000",
  "1000000", "2000000"};

I've added the missing 74880, 250000, and also 31250 (that is not available in the Arduino IDE 1.8.x).

[cmaglie]

test builds in https://github.com/arduino/serial-monitor/actions/runs/3591605334

[mariuszdec]

Hi all,
those new baudrates are importatnt to me.
I have just downloaded 2.03 dated on 15-th December but there is not available yet.
I understand that we are waiting for umbynos.
Umberto please review this solution.
Best wishes for a new 2023-th year :)
Regards
Mariusz

[per1234]

Thanks Cristian!

[MatteoPologruto]

I tested the changes using the sketch SerialEcho, with an Arduino Uno board on Windows 11. The only baudrate value that is giving me some issues is 300, while the others work fine both on the arduino-cli and on the Java Ide. The issues might be related to the board, but I'm not entirely sure.

[facchinm]

I tested the changes using the sketch SerialEcho, with an Arduino Uno board on Windows 11. The only baudrate value that is giving me some issues is 300, while the others work fine both on the arduino-cli and on the Java Ide. The issues might be related to the board, but I'm not entirely sure.

@MatteoPologruto 300bps is a special baud that might not work on UNO/Mega (due to shortcomings in the usb2serial chip firmware), so the failure is "expected"