Slightly alter star appearence time and full brightness time (#7921)

Slightly alter star appearence time and full brightness time (#7921)

At sunset:
Stars first appear slightly later, at the time the sun disappears over the horizon,
this fixes seeing dark stars in front of the sun horizon glow texture.
Stars reach full brightness slightly earlier at time 20000, not so excessively long
after sunset.

The above behaviour is also applied at sunrise, but of course, time-inverted.