Fix copyright date display in About KDevelop/KDevPlatform dialogs

Passing in a plain int gets localized integer text rendering with
e.g. thousand separator (2.019), which is not what we want here.

By feedback of localization teams the original year span is used, and
not mapped into local calendar systems at least by the active teams,
so just passing in a plain string is fine for now.
Cmp. https://marc.info/?l=kde-i18n-doc&m=157238863619132&w=2

BUG:413390
FIXED-IN:5.4.4