[Sidebar] Use new Kirigami ListSectionHeader component
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:
Reviewed By: VDG, GB_2
Subscribers: plasma-devel
Tags: Plasma
Maniphest Tasks: T10384
Differential Revision: https://phabricator.kde.org/D23052