Investigate why packages are held back, and stop release upgrade

I have a System76 computer that came with Ubuntu, but I installed xfce4 on it, and made other changes so that it is now effectively Xubuntu. It has been performing very well, but will now not upgrade from Focal to Jammy. It complains:

  fwupd-signed gnome-software gnome-software-common gnome-software-plugin-snap ubuntu-software

I have tried solutions from other questions that involved apt --upgrade-only, but they did not help. The rest were hard to follow, and I was not sure that trying them would be safe, so I'm doing this as a new question.

The first question probably should be: do I need gnome-software and ubuntu-software when I'm using lightdm and xfce?

After reading answer 1, I made some changes and retried. It failed like so...

Building dependency tree          
Reading state information... Done

Calculating the changes

Calculating the changes

Error in sys.excepthook:
Traceback (most recent call last):
  File "/tmp/ubuntu-release-upgrader-5218svyf/DistUpgrade/DistUpgradeApport.py", line 49, in apport_crash
    from apport.report import Report
  File "/usr/lib/python3/dist-packages/apport/__init__.py", line 5, in <module>
    from apport.report import Report
  File "/usr/lib/python3/dist-packages/apport/report.py", line 32, in <module>
    import apport.fileutils
  File "/usr/lib/python3/dist-packages/apport/fileutils.py", line 495, in <module>
    def get_process_environ(proc_pid_fd: int) -> dict[str, str]:
TypeError: 'type' object is not subscriptable

Original exception was:
Traceback (most recent call last):
  File "/tmp/ubuntu-release-upgrader-5218svyf/jammy", line 8, in <module>
    sys.exit(main())
  File "/tmp/ubuntu-release-upgrader-5218svyf/DistUpgrade/DistUpgradeMain.py", line 241, in main
    if app.run():
  File "/tmp/ubuntu-release-upgrader-5218svyf/DistUpgrade/DistUpgradeController.py", line 2042, in run
    return self.fullUpgrade()
  File "/tmp/ubuntu-release-upgrader-5218svyf/DistUpgrade/DistUpgradeController.py", line 1991, in fullUpgrade
    if not self.calcDistUpgrade():
  File "/tmp/ubuntu-release-upgrader-5218svyf/DistUpgrade/DistUpgradeController.py", line 1100, in calcDistUpgrade
    if not self.cache.installTasks(self.tasks):
AttributeError: 'DistUpgradeController' object has no attribute 'tasks'
=== Command terminated with exit status 1 (Sat Sep  7 18:27:32 2024) ===

It seems to have gotten as far as processing triggers. Now what?

Being advised that my original question was answered, I opened a new one to investigate why I get a crash when I try to update. See Why does do-release-upgrade fail with an error on Xubuntu 20.04.6 -> 22.04?