update deskutils/fet to 5.31.5

Changes (from http://lalescu.ro/liviu/fet/news.html ):

• Speed improvement in the precomputation if using many rooms and many buildings
• Bug fix in the printout of the subjects' timetables
• Rare crash bugs fixed
• Updated arabic translation

PR: 218787
Submitted by: Zsolt Udvari <udvzsolt@gmail.com> (maintainer)