ui.rc files: consistenly use <gui> instead of deprecated <kpartgui>
<kpartgui>
as root element has been deprecated since < KF 5.0
See also https://invent.kde.org/frameworks/kxmlgui/-/blob/master/src/kpartgui.dtd#L32
<kpartgui>
as root element has been deprecated since < KF 5.0
See also https://invent.kde.org/frameworks/kxmlgui/-/blob/master/src/kpartgui.dtd#L32