deskutils/fet: Update to 6.9.0/Add flavors qt5 and qt6
- qt6 as default flavor as recommended by upstream
- Remove ONLY_CL option: qt6 flavor needs only one package so the ONLY_CL option is useless in qt6 (the number of dependencies don't decrease). I suggest to remove it because (as I understand) qt5 will be outdated soon.
ChangeLog: https://lalescu.ro/liviu/fet/news.html
PR: 270690