Setting `qt.platformTheme.name = "gtk"` silently rewrote the
QT_QPA_PLATFORMTHEME session variable to `gtk2` and pulled in the
qtstyleplugins and qt6gtk2 packages, which is not what users asked for
and relies on unmaintained packages. There is no `gtk` Qt platform
theme, so `gtk` now maps to Qt's built-in `gtk3` platform-theme plugin
and installs no extra packages.
Users who want the previous behavior can set
`qt.platformTheme.name = "gtk2"` explicitly, which keeps
QT_QPA_PLATFORMTHEME=gtk2 and installs qtstyleplugins and qt6gtk2. The
`qt.useGtkTheme` -> `qt.platformTheme` migration now maps the legacy
option to `{ name = "gtk2"; }` so existing configs are unaffected.
Fixes#8663
153bad8fb5