Use small font for ExpandableListItem subtitle

Summary:
We're moving towards using the small font for subtitles in Kirigami BasicListItem (D29432)
so we should probably do the same here since it has the same title + subtitle layout.

Test Plan:
Before:

After:

Reviewers: Plasma, VDG, broulik, ndavis

Reviewed By: VDG, ndavis

Subscribers: cblack, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D29434