deskutils/fet: update to 6.7.7

ChangeLog: https://lalescu.ro/liviu/fet/news.html

• Interface improvement: in the activities dialog, in the list of time constraints directly related to the selected activity, there will be shown the related constraints of type teacher/students set not available times.
• Minor improvements.
• Code cleanup.

PR: 268443
Reported by: uzsolt@uzsolt.hu (maintainer)