LoadingPage: Use Kirigami.Heading
Summary: Unifies the look of the loading message with the other messages in discover.
Test Plan: Font size is correct when installing a package e.g from a flatpak ref.
Reviewers: Discover Software Store, ngraham
Reviewed By: Discover Software Store, ngraham
Subscribers: ngraham, plasma-devel
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D20675