As part of T278905: Reconsider which mailman3 version we're running, I backported the mailman3 packages from bullseye to buster and we are ready to install them on lists-next. This will apply schema changes and needs DBA supervision/coordination, as previously discussed.
The specific change to be merged is https://gerrit.wikimedia.org/r/678134 and then follow https://wikitech.wikimedia.org/wiki/Mailman#Upgrading_Mailman3
DBA please let me know when you can supervise/watch over this, we're ready to go whenever.