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] nextcloud/server#24836

nixos/modules/services/web-apps/nextcloud.nix

@@ -446,7 +446,13 @@ in {
       dbtableprefix = mkOption {
         type = types.nullOr types.str;
         default = null;
-        description = "Table prefix in Nextcloud's database.";
+        description = ''
+          Table prefix in Nextcloud's database.
+
+          __Note:__ since Nextcloud 20 it's not an option anymore to create a database
+          schema with a custom table prefix. This option only exists for backwards compatibility
+          with installations that were originally provisioned with Nextcloud <20.
+        '';
       };
       adminuser = mkOption {
         type = types.str;
@@ -826,6 +832,13 @@ in {
           Using config.services.nextcloud.poolConfig is deprecated and will become unsupported in a future release.
           Please migrate your configuration to config.services.nextcloud.poolSettings.
         '')
+        ++ (optional (cfg.config.dbtableprefix != null) ''
+          Using `services.nextcloud.config.dbtableprefix` is deprecated. Fresh installations with this
+          option set are not allowed anymore since v20.
+
+          If you have an existing installation with a custom table prefix, make sure it is
+          set correctly in `config.php` and remove the option from your NixOS config.
+        '')
         ++ (optional (versionOlder cfg.package.version "25") (upgradeWarning 24 "22.11"))
         ++ (optional (versionOlder cfg.package.version "26") (upgradeWarning 25 "23.05"))
         ++ (optional (versionOlder cfg.package.version "27") (upgradeWarning 26 "23.11"))

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";