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.
The first version supporting PHP 8.0 is Nextcloud 21. Therefore we can add migrations only to Nextcloud 21 forward, and so we only support migrating from Nextcloud 20 (Mail-in-a-Box versions v0.51+). Migration steps through Nextcloud 21 and 22 are added.
Also:
* Fix PHP APUc settings to be before Nextcloud tools are run.
* Add the PHP PPA.
* Specify the version when invoking the php CLI.
* Specify the version in package names.
* Update paths to 8.0 (using a variable in the setup scripts).
* Update z-push's php-xsl dependency to php8.0-xml.
* php-json is now built-into PHP.
Although PHP 8.1 is the stock version in Ubuntu 22.04, it's not supported by Nextcloud yet, and it likely will never be supported by the the version of Nextcloud that succeeds the last version of Nextcloud that supports PHP 7.2, and we have to install the next version so that an upgrade is permitted, so skipping to PHP 8.1 may not be easily possible.
See [mailinabox issue #2088](https://github.com/mail-in-a-box/mailinabox/issues/2088). This also updates the commit hashes to for anyone updating from NextCloud version 17 (as shown in the related issue) since a different hash is used for tags vs releases.
This was tested and verified to work on a setup previously running v0.44 and then updating to the latest version (v56).
