Version 3.14 was recently released with security fixes, and GitHub
runners have been updated to use it. Don't hardcode the version in
Program Files directory name to support any 3.x. Since 3.x support will
be dropped within a year, need to plan migration to 5.x when it's
released (4.x isn't suitable because of missing heat replacement).
Bring back old configuration that was using Qt 5 and split Qt build in
two to support both Qt 5 and Qt 6. Last Qt 5 builds were using 5.14.2,
bump that to latest 5.15.8.
Qt 6 only supports Windows 10+, so adjust the system version check
accordingly. Not bumping Qt 6 version (although 6.4.2 is available)
while we're still in patch release mode.
Building a whole new package is not nice since Qt client is only one
part of it, the rest doesn't require newer OS version. Nevertheless,
it's a quick and easy fix.
