Use new Kirigami ListSectionHeader component on Updates and Sources pages

Summary:
This allows all the section headers to always have the same look and feel and be
adjusted in just one place.

Depends on D23049

Test Plan:
Updates page:

Sources page: no visual or functional changes

Reviewers: Discover Software Store, VDG, apol, GB_2

Reviewed By: Discover Software Store, VDG, apol, GB_2

Subscribers: plasma-devel

Tags: Plasma

Maniphest Tasks: T10384

Differential Revision: https://phabricator.kde.org/D23057