Autobuild: Windows: Build with Qt 6.4.3 on 64bit

32bit builds remain on 5.15.2, as Qt6 is not available there.
Note: CI builds will now drop unofficial 64 Bit Windows 7 and 8.1
support. Since both are no longer supported by Microsoft, this should
not be an issue. Users should update their OS or compile from source.

.github/autobuild/windows.ps1

@@ -20,7 +20,7 @@ $DownloadCacheDir = 'C:\AutobuildCache'
 # The following version pinnings are semi-automatically checked for
 # updates. Verify .github/workflows/bump-dependencies.yaml when changing those manually:
 $Qt32Version = "5.15.2"
-$Qt64Version = "5.15.2"
+$Qt64Version = "6.4.3"
 $AqtinstallVersion = "3.1.5"
 $JackVersion = "1.9.22"
 $Msvc32Version = "win32_msvc2019"