Show a new version string description for updates where the old and new versions are identical

Summary:
This is common for Flatpak runtime updates, where it seems weird to show "3.34 -> 3.34".

With this patch, instead Discover will show "Update to version 3.34", which is more
accurate.

Test Plan:
No change for updates where old and new are different or only new version is available

Reviewers: VDG, Discover Software Store, apol

Reviewed By: Discover Software Store, apol

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D24005