nixos/nextcloud: implement a safe upgrade-path between 19.09 and 20.03

It's impossible to move two major-versions forward when upgrading
Nextcloud. This is an issue when comming from 19.09 (using Nextcloud 16)
and trying to upgrade to 20.03 (using Nextcloud 18 by default).

This patch implements the measurements discussed in #82056 and #82353 to
improve the update process and to circumvent similar issues in the
future:

* `pkgs.nextcloud` has been removed in favor of versioned attributes
  (currently `pkgs.nextcloud17` and `pkgs.nextcloud18`). With that
  approach we can safely backport major-releases in the future to
  simplify those upgrade-paths and we can select one of the
  major-releases as default depending on the configuration (helpful to
  decide whether e.g. `pkgs.nextcloud17` or `pkgs.nextcloud18` should be
  used on 20.03 and `master` atm).

* If `system.stateVersion` is older than `20.03`, `nextcloud17` will be
  used (which is one major-release behind v16 from 19.09). When using a
  package older than the latest major-release available (currently v18),
  the evaluation will cause a warning which describes the issue and
  suggests next steps.

  To make those package-selections easier, a new option to define the
  package to be used for the service (namely
  `services.nextcloud.package`) was introduced.

* If `pkgs.nextcloud` exists (e.g. due to an overlay which was used to
  provide more recent Nextcloud versions on older NixOS-releases), an
  evaluation error will be thrown by default: this is to make sure that
  `services.nextcloud.package` doesn't use an older version by accident
  after checking the state-version. If `pkgs.nextcloud` is added
  manually, it needs to be declared explicitly in
  `services.nextcloud.package`.

* The `nixos/nextcloud`-documentation contains a
  "Maintainer information"-chapter  which describes how to roll out new
  Nextcloud releases and how to deal with old (and probably unsafe)
  versions.

Closes #82056