Drop Windows 8.1 from CI verification

As QTQAINFRA-1390 suggests to drop MSVC 2013 from CI verification, it would also drop Windows 8.1 coverage. That MSVC 2013 is the sole configuration verified with Windows 8.1.
If we wanted to keep Windows 8.1, we have to move something to it. Windows 7 has only MinGW builds, but Windows 10 has both MSVC 2015 and 2017. If we move MSVC 2015 to Windows 8.1, we need to create a new Tier1 image for that.

So two proposals to select from:
a) Drop Windows 8.1
b) Keep it and move MSVC 2015 to it