[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:**
{F6520053}

**After:**
{F6520055}

**//Plasma defaults testing//**

**Before:**
{F6520791}

**After:**
{F6520792}

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