Update Qtel to Qt6

[AsciiWolf]

It would be great if Qtel could be updated to Qt6. Qt5 is becoming obsolete and will be removed from repositories of many Linux distributions in upcoming years.

[dl1jbe]

Speaking as a Gentoo distro maintainer for svxlink I would like to second that.

We are in the process of moving away from Qt5 in the next month (see
https://bugs.gentoo.org/948836 for some background) and it would be good to have an Qt6 ready alternative.