Qt::DefaultLocaleShortDate / QLocale::ShortFormat date parsing doesn't deal with 4-digit years

QDate::fromString(QLatin1String("2/3/2020"), Qt::DefaultLocaleShortDate) doesn't work. QDate::fromString(QLatin1String("2/3/20"), Qt::DefaultLocaleShortDate) parses the date as Feb 3, 1920 in an American locale. Yet QLocale().dateFormat(QLocale::ShortFormat) recommends "M/d/yy" as the format to use. The idea of using 2-digit years at all was obsolete more than 20 years ago.

In Qt 6 we should always use 4-digit years for reading and writing short-format dates.

If we must support 2-digit years for some reason, we could use a sliding window somehow so that 99 means 1999 and 20 means 2020, but that's a slippery slope to extended bikeshedding about where the boundary should be.

$ ./dateparse 02/03/99
[     0.000 D] default unknown - acceptable short format "M/d/yy"
[     0.000 D] default unknown - parsed "02/03/99" as "Wed Feb 3 1999"
[     0.000 D] default unknown - parsed 2/3/2020 as ""
[     0.000 D] default unknown - parsed 2/3/20 as "Tue Feb 3 1920"