Fix failing CI: remove deprecated runners, fix Qt6 WebEngine build

[Copilot]

CI was broken by two independent issues: legacy jobs targeting removed GitHub runner images (ubuntu-20.04, macos-10.15, windows-2019), and a Qt6 build failure where QWebEnginePage moved from QtWebEngineWidgets to QtWebEngineCore.

official.yml

• Drop legacy Qt 4.8 packaging jobs (ubuntu-20.04, macos-10.15, windows-2019 runners are gone/deprecated)
• Add ci-qt-webkit job on ubuntu-24.04 with qt6-webengine-dev that builds/tests/lints with --features qt-webkit

crates/core/build.rs

• Add .qt_module("WebEngineCore") — Qt6 moved QWebEnginePage/QWebEngineSettings out of QtWebEngineWidgets
• Fix cxx-qt-build 0.8 API: .file() for the Rust bridge (qt_webkit.rs), .cpp_file() for the C++ impl
• Add Gui and Widgets modules for QImage/QPixmap and QApplication
• Wrap cc_builder call in unsafe as required by cxx-qt-build 0.8

cxx_qt_build::CxxQtBuilder::new()
    .file("src/qt_webkit.rs")
    .cpp_file("src/webkit_renderer.cpp")
    .cc_builder(|cc| { cc.include("include"); })
    .qt_module("Gui")
    .qt_module("Widgets")
    .qt_module("WebEngineCore")   // QWebEnginePage lives here in Qt6
    .qt_module("WebEngineWidgets")
    .build();

webkit_renderer.h / webkit_renderer.cpp

• Add const to run_scripts parameter (rust::Slice<const rust::Str>) to match cxx-generated signature for &[&str]