[Notifications] Use new Kirigami ListSectionHeader component in KCM's sources page

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: {F7181763}

Reviewers: #plasma, #vdg, GB_2, broulik

Reviewed By: #vdg, GB_2

Subscribers: broulik, plasma-devel

Tags: #plasma

Maniphest Tasks: T10384

Differential Revision: https://phabricator.kde.org/D23054