nixos/nextcloud: update upgrade-path for nextcloud19

Enhance the heuristics to make sure that a user doesn't accidentally
upgrade across two major versions of Nextcloud (e.g. from v17 to v19).

The original idea/discussion has been documented in the nixpkgs manual[1].

This includes the following changes:

* `nextcloud19` will be selected automatically when having a stateVersion
  greater or equal than 20.09. For existing setups, the package has to
  be selected manually to avoid accidental upgrades.

* When using `nextcloud18` or older, a warning will be thrown which recommends
  upgrading to `nextcloud19`.

* Added a brief paragraph about `nextcloud19` in the NixOS 19.09 release
  notes.

* Restart `phpfpm` if the Nextcloud-package (`cfg.package`) changes[2].

[1] https://nixos.org/nixos/manual/index.html#module-services-nextcloud-maintainer-info
[2] https://github.com/NixOS/nixpkgs/pull/89427#issuecomment-638885727