Fix Flatpak build issues: EOL runtime and icon naming mismatch

- Bump the KDE runtime/SDK from 6.8 (now end-of-life per Flathub) to the
  current stable 6.10.
- The .desktop file's Icon=orbithub didn't match the app ID
  (io.orbithub.OrbitHub), so Flatpak's export step skipped it entirely
  ("Icon referenced in desktop file but not exported"). Use the app-ID-
  matching icon name everywhere instead, and drop the now-redundant
  renamed orbithub.svg install() rule that existed only for the old name.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

CMakeLists.txt

@@ -236,11 +236,6 @@ install(FILES
     packaging/linux/io.orbithub.OrbitHub.svg
     DESTINATION ${CMAKE_INSTALL_DATADIR}/icons/hicolor/scalable/apps
 )
-install(FILES
-    packaging/linux/io.orbithub.OrbitHub.svg
-    DESTINATION ${CMAKE_INSTALL_DATADIR}/icons/hicolor/scalable/apps
-    RENAME orbithub.svg
-)
 install(FILES
     packaging/linux/io.orbithub.OrbitHub.metainfo.xml
     DESTINATION ${CMAKE_INSTALL_DATADIR}/metainfo