nixos/nextcloud: drop default for dbtype setting

Closes #355606

To quote the system requirements[1]:

> Database: SQLite 3.16+ (only recommended for testing and minimal-instances)

As discussed[2], this would already fail at eval-time for everyone and
makes all sqlite users aware that this is probably not the best choice.
Doing that regardless, is now an explicit decision.

[1] https://docs.nextcloud.com/server/30/admin_manual/installation/system_requirements.html
[2] https://github.com/NixOS/nixpkgs/pull/369242#discussion_r1899706512

nixos/doc/manual/release-notes/rl-2505.section.md

@@ -111,6 +111,11 @@
 
 - `ps3-disc-dumper` was updated to 4.2.5, which removed the CLI project and now exclusively offers the GUI
 
+- [](#opt-services.nextcloud.config.dbtype) is unset by default, the previous default was `sqlite`.
+  This was done because `sqlite` is not a reasonable default since it's
+  [not recommended by upstream](https://docs.nextcloud.com/server/30/admin_manual/installation/system_requirements.html)
+  and thus doesn't qualify as default.
+
 - `asusd` has been upgraded to version 6 which supports multiple aura devices. To account for this, the single `auraConfig` configuration option has been replaced with `auraConfigs` which is an attribute set of config options per each device. The config files may also be now specified as either source files or text strings; to account for this you will need to specify that `text` is used for your existing configs, e.g.:
   ```diff
   -services.asusd.asusdConfig = '''file contents'''

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

@@ -416,7 +416,6 @@ in {
     config = {
       dbtype = mkOption {
         type = types.enum [ "sqlite" "pgsql" "mysql" ];
-        default = "sqlite";
         description = "Database type.";
       };
       dbname = mkOption {

nixos/tests/nextcloud/basic.nix

@@ -59,6 +59,7 @@ runTest (
 
           services.nextcloud = {
             enable = true;
+            config.dbtype = "sqlite";
             datadir = "/var/lib/nextcloud-data";
             autoUpdateApps = {
               enable = true;

nixos/tests/nextcloud/with-objectstore.nix

@@ -36,6 +36,8 @@ runTest (
           networking.firewall.allowedTCPPorts = [ 9000 ];
           environment.systemPackages = [ pkgs.minio-client ];
 
+          services.nextcloud.config.dbtype = "sqlite";
+
           services.nextcloud.config.objectstore.s3 = {
             enable = true;
             bucket = "nextcloud";