Reduce visual strength of version numbers on Updates page

Summary:
This patch improves the visual hierarchy and scannability of the updates list by
reducing the opacity (and hence visual strength) of the version numbers.

In conjunction with D23057, the Updates list will look like the list in KWin's
Desktop Effects KCM, increasing consistency.

Test Plan:
Before: {F7181844}
After: {F7181843}

Reviewers: #vdg, #discover_software_store, apol

Reviewed By: #discover_software_store, apol

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D23058