[KWidgetsAddons] Do not use light font styles for headings (3/3)

Summary:
This patch brings back regular font styles for headings to KTitleWidget for the sake of consistency with QML and Kirigami, where they were also reintroduced in favor of better legibility.

CCBUG: 402730

FIXED-IN: 5.54

Test Plan:
Before:

Reviewers: ngraham, Frameworks, VDG, Plasma

Reviewed By: ngraham, VDG

Subscribers: cfeck, filipf, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D17907