Reproducer attached.

A performance issue with QDateTime::fromString.

The customer reported :
 - The function has slowed down drastically between Qt 5.15.x (the previous Qt release the customer used) and Qt 6.6.0.
 - this issue was monitored only with Windows.
 - based on the customer's profiling, it seems that most of the time is spent in QWinTimeZonePrivate::availableWindowsIds function.
 - Running the code on the customer's laptop takes ~100ms with Qt 5.15 and ~10000ms with Qt 6.6.

The same issue was reproduced at my laptop as well, the result was as follows:

  Qt 6.6.1:
    XXX - 10000 x QDateTime::fromString took 4965 milliseconds

  Qt 5.15.16
    XXX - 10000 x QDateTime::fromString took 49 milliseconds