Update bootloaders

I get that that your QA is [infinite](https://github.com/arduino/ArduinoCore-avr/pull/24#issuecomment-441559730), that you want to care about only [production](https://github.com/arduino/ArduinoCore-avr/issues/150#issuecomment-366689374) models, and that there's much else ado too (in fact, updating avrdude is also sorely in need imho, and it would in turn certainly come handy for this issue)

But frankly enough, optiboot wasn't exactly the most polished gem on the shed back in the days, and after almost a decade from the [last version](https://github.com/arduino/ArduinoCore-avr/tree/master/bootloaders) bump the amount of *shortcomings* that have been piling up here is just huge (my pet peeve is reading EEPROM [returning](https://github.com/avrdudes/avrdude/issues/1227#issuecomment-1356813293) instead the flash memory, but I suppose that 2023 could also be the year of auto-baud not hurting either).

IMHO you should either update it (I [believe](https://github.com/Optiboot/optiboot/issues/235#issuecomment-1547016697) that versions with all features enabled in just 512B should exist) or switch to [Urboot](https://github.com/stefanrueger/urboot).

Start by inspecting the bootloaders/ tree and the linked Optiboot and Urboot projects, including the referenced EEPROM and auto-baud concerns. Determine which production AVR models and bootloader choice are in scope; done requires an agreed update or replacement plan rather than the broad alternatives currently listed.

Written by the indexing model from the issue text.