nixos/nextcloud: deprecated dbtableprefix

Closes #320381

Installation with a custom dbtableprefix is not allowed anymore for a
while[1] and we shouldn't advertise it as such.

The option is deprecated for now since I'm not sure if there are some
weird corner-cases where removing the option directly would break
existing installations from before <20 with a custom dbtableprefix. The
migration-path for such a case is as follows:

* Check if /var/lib/nextcloud/config/config.php has the correct
  dbtableprefix set and if not, take care of it.
* Remove `dbtableprefix` from the NixOS configuration. It's effectively
  state anyways.

After a bit of time to switch (perhaps after the next release
branchoff), the option can be removed.

[1] https://github.com/nextcloud/server/issues/24836

nixos/tests/nextcloud/basic.nix

@@ -38,7 +38,6 @@ runTest ({ config, ... }: {
       services.nextcloud = {
         enable = true;
         datadir = "/var/lib/nextcloud-data";
-        config.dbtableprefix = "nixos_";
         autoUpdateApps = {
           enable = true;
           startAt = "20:00";