Use font.weight instead of font.bold

font.bold is a pseudo property only forwarding to and reading from font.weight in Qt code and since the qqc2-desktop-style Label
sets font.weight this seems to have precedence over our font.bold definition.

I don't think that's a Kirigami but rather a Qt bug. The same happens with PlasmaComponents.Label.

Differential Revision: https://phabricator.kde.org/D11764