Flatpak: Bump to platform 9

[ryonakano]

Changes Summary

• Flatpak: Bump to platform 9
• libical: Update libical-3.0.16.tar.gz to 3.0.20 to fix the following build error with platform 9

========================================================================
Building module libical in /__w/tasks/tasks/.flatpak-builder/build/libical-1
========================================================================
CMake Error at CMakeLists.txt:83 (cmake_minimum_required):
  Compatibility with CMake < 3.5 has been removed from CMake.

  Update the VERSION argument <min> value.  Or, use the <min>...<max> syntax
  to tell CMake that the project requires at least <min> but has been updated
  to work with policies introduced by <max> or earlier.

  Or, add -DCMAKE_POLICY_VERSION_MINIMUM=3.5 to try configuring anyway.


-- Configuring incomplete, errors occurred!
Error: module libical: Child process exited with code 1
Error: Build failed: Error: The process '/app/bin/xvfb-run' failed with exit code 1

Checklist

• Confirmed the app launches on Flatpak