plugins/nightlight: Relax custom times constraints

Vlad Zahorodniirequested to merge
work/zzag/backport-custom-times-fix into Plasma/6.1

Apparently the night light kcm allows to set the custom times so the evening is earlier than the morning to handle extreme cases close to the North and the South pole.

NightLightManager::updateTransitionTimings() should require no changes.

BUG: 489366 (cherry picked from commit 2adf9624)

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