net-im/telegram-desktop: Remove Qt flavors

Drop local Qt 5.x support for ease of maintainership and its eventual
removal from the FreeBSD ports tree. Qt 5.x has been long deprecated
upstream [1] and the FreeBSD kde@ team recommends using Qt 6.x for
end-user applications whenever possible.

[1] https://www.qt.io/blog/qt-5.15-support-ends

Bump PORTREVISION.