ChangeLog: https://nextcloud.com/changelog/#latest21
* Packaged 21.0.0, test-deployed it to my personal instance and tested
the most basic functionality (`davfs2`-mount, {card,cal}dav sync, file
management).
* Bumped the default version for unstable/21.05 to `nextcloud21`. Since
`nextcloud20` was added after the release of 20.09 (and thus the
default on 20.09 is still `nextcloud19`), it's now needed to upgrade
across two majors.
This is not a problem though since it's possible to upgrade to v20 on
20.09 already and if not, the module will guard the administrator
through the upgrade with eval warnings as it's the case since 20.03.
* Dropped `nextcloud17` attribute and marked `nextcloud18` as EOL.
f7011c70f3