Merge: nixos/nextcloud: configure redis by default (#421813)

- `services.dnscrypt-proxy2` gains a `package` option to specify dnscrypt-proxy package to use.

- `services.nextcloud.configureRedis` now defaults to `true` in accordance with upstream recommendations to have caching for file locking. See the [upstream doc](https://docs.nextcloud.com/server/31/admin_manual/configuration_files/files_locking_transactional.html) for further details.

- `services.gitea` supports sending notifications with sendmail again. To do this, activate the parameter `services.gitea.mailerUseSendmail` and configure SMTP server.

- `libvirt` now supports using `nftables` backend.

    configureRedis = lib.mkOption {

      type = lib.types.bool;

      default = config.services.nextcloud.notify_push.enable;

      defaultText = lib.literalExpression "config.services.nextcloud.notify_push.enable";

      default = true;

      description = ''

        Whether to configure Nextcloud to use the recommended Redis settings for small instances.

        ::: {.note}

        The Nextcloud system check recommends to configure either Redis or Memcache for file lock caching.

        :::

        ::: {.note}

        The `notify_push` app requires Redis to be configured. If this option is turned off, this must be configured manually.

        :::

          services.nextcloud = {

            caching = {

              apcu = true;

              redis = false;

              memcached = true;

            };

            config.dbtype = "mysql";

            configureRedis = false;

          };

          services.memcached.enable = true;