[kirigami] Do not use light font styles for headings (2/3)

Summary:
This patch turns off light font styles for some headings because they cause visual and legibility issues, as explained in the respective bug report.

CCBUG: 402730

FIXED-IN: 5.54

Test Plan:
Worst case scenario testing - font does not have a "light" variant and falls back to using a thinner one such as "hairline"

Before:

After:

Plasma defaults testing

Before:

After:

Reviewers: ngraham, Plasma, VDG, Frameworks, mart

Reviewed By: ngraham, Plasma, VDG, mart

Subscribers: fabianr, rooty, Frameworks, ngraham, Plasma, VDG, abetts, mart, plasma-devel

Tags: Kirigami

Differential Revision: https://phabricator.kde.org/D17906