Change the default SMTP port to `25`, to better match the default
address `localhost`. This gets rid of some error outputs in the test,
where it fails to connect to localhost:465.
Also, don't enable postfix by default unless it's actually useful to
us.
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.
The NixOS 21.03 release has been delayed to 21.05. See NixOS/rfcs#80.
There are two instances of 21.03 which have been left as is, since they
are in stateVersion comparisons. This will ensure that existing user
configurations which refer to 21.03 will continue to work.
