Releases: Add Qt6 releases for macOS/Windows

[bugwelle]

Will (hopefully) also fix #429

TODO:

• [ ] automate Qt6 release via scripts (as we do for Qt5)
  • [x] macOS => #1491
    • [ ] change required minimum version for the Qt6 build
  • [x] Windows => https://github.com/Komet/MediaElch/pull/1492
• [ ] update Download page to tell users which version to download (Windows 7 - 8: Qt5, Windows 10: Qt6)
• [ ] test it
• [x] check resulting bundle sizes
  • [x] macOS (needs hack)
  • [x] Windows (+ 3MiB in ZIP file)
• [x] Test QML file