Revert Qt 6 migration of kiosk browser

[knuton]

The Qt 6 version of QtWebEngine brought too many sneaky breaking changes and is holding up this PlayOS release because of it.

Revert to unblock releasing the other features and component updates. Migration can be picked up without having to rush it.

Blocking issues

The last currently known blocking issue is broken session persistence with the Qt 6 kiosk, due to two reasons:

1. Breaking change in Qt 6, default profile is now volatile and never persisted
2. Change in wrapper setup kiosk/default.nix seems to have changed filepath at which the default profile is stored (.kiosk-browser-wrapped instead of kiosk-browser)

It is also currently unconfirmed whether the Qt 6 QtWebEngine is going to pick up the Qt 5 profile information, and if so, whether it is backwards compatible when booting back into Qt 5 afterwards.

Checklist

• Changelog updated
• Code documented
• User manual updated

[guyonvarch]

Looking good:

• looking at the last merges, the revert list fits,
• I could run the VM and play a game,
• I can refresh and hard refresh, and I can still tab to have the focus.

[knuton]

Some links related to the profile issue:

• https://forum.qt.io/topic/131277/qtwebengine-cannot-save-cache (ephemeral profile)
• Switching to Qt 6 by default qutebrowser/qutebrowser#7202 (backwards compat)