Nextcloud:
* The Nextcloud user_external 1.0.0 package for Nextcloud 21.0.7 isn't available from Nextcloud's releases page, but it's not needed in an intermediate upgrade step (hopefully), so we can skip it.
* Nextcloud updgrade steps should not be elifs because multiple intermediate upgrades may be needed.
* Continue if the user_external backend migration fails. Maybe it's not necessary. It gives a scary error message though.
* Remove a line that removes an old file that hasn't been in use since 2019 and the expectation is that Ubuntu 22.04 installations are on fresh machines.
Backups:
* For duplicity, we now need boto3 for AWS.
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