The Mailman3 mailing list forwarding, administration, and archive display all worked here under Bullseye. The symptom I see now is that when I try to view a mailing list archive with a web browser, I see a spinning star where I previously saw data.
In addition to the package upgrades, I have done the 'make migrations' and 'migrate' steps. I have modified /usr/lib/python3/dist-packages/xapian_backend.py, replacing 'from django.utils import six' with 'import six'. 'python3 manage.py check' finds no issues. I have defined "DEFAULT_AUTO_FIELD = 'django.db.models.BigAutoField'". After the changes described in the previous paragraph, /var/log/mailmain3/mailman.log, /var/log/mailman3/web/mailman-web.log, and /var/log/apache2/error.log are free of any errors or warnings. It is very possible that I have made an error during the upgrade process, or that there is an additional step needed that I have not performed. Unfortunately, at this point, I do not know where to look for additional clues. Suggestions about where to look or how to get additional information would be most welcome. Ken