chore: silence some deprecation warnings in Qt 6.8

[Nerixyz]

The underlying deprecations aren't new, but they haven't shown up as a warning in 6.7 and below.

Notably, the variant conversion/checking in controllers/filters/lang/Types.hpp was quite vocal (seems to be included in most files; transitively). QVariant::type is deprecated (typeId is preferred) in Qt 6, but there's no alternative in Qt 5. Furthermore, QMetaType::Type contains predefined type-ids, while a type-id is of type int (in both Qt 5 and 6). This isn't really relevant for us, since we don't use user-types.

Some other changes:

• QVariant::canConvert<T> is preferred over QVariant::canConvert(type-id)
• QString::fromUcs4 prefers the char32_t overload (available in Qt 5)
• Prefer std::as_const over qAsConst (but we don't need either)
• QRegularExpression got a matchView in 6.5 which is preferred over match for string views to avoid allocating a string
• QDateTime::setTimeSpec is deprecated (use setTimeZone - that's what setTimeSpec does under the hood too)

There are two other deprecations that are now reported which don't have a Qt 5 equivalent:

• QMouseEvent::{screenPos, globalPos} are deprecated, and globalPosition should be used (not in Qt 5)
• QWidget::addAction: use addAction(text, shortcut, object, slot) instead (not in Qt 5 and these methods are on QMenu in Qt 5)

Otherwise, Qt 6.8 seems to work good enough on Windows. The only compilation error was that QFile(path) is now explicit (fixed in https://github.com/Chatterino/chatterino2/pull/5527/files#diff-07c979414bf7268bd34c32515e8500e25726e4ed1d43eb970f45bacff76b5bf3R111).