net-p2p/tremotesf: update to 2.9.0 and de-flavorize

Upstream removed Qt 5 support with this release. Despite removing
flavors, "-qt6" suffix remains to comply with POLA.

Additionally, adjust description to avoid confusion as other modern
GUIs may exist too.

Changelog: https://github.com/equeim/tremotesf2/blob/2.9.0/CHANGELOG.md

PR: 290040
Reported by: Anton Saietskii <vsasjason@gmail.com> (maintainer)