jellyfin_10_5: remove unmaintained version

This version contains a vulnerability[1], and isn't maintained. The
original reason to have two jellyfin versions was to allow end-users to
backup the database before the layout was upgraded, but these backups
should be done periodically.

[1]: <https://nvd.nist.gov/vuln/detail/CVE-2021-21402>

nixos/modules/services/misc/jellyfin.nix

@@ -18,6 +18,7 @@ in
 
       package = mkOption {
         type = types.package;
+        default = pkgs.jellyfin;
         example = literalExample "pkgs.jellyfin";
         description = ''
           Jellyfin package to use.
@@ -98,11 +99,6 @@ in
       };
     };
 
-    services.jellyfin.package = mkDefault (
-      if versionAtLeast config.system.stateVersion "20.09" then pkgs.jellyfin
-        else pkgs.jellyfin_10_5
-    );
-
     users.users = mkIf (cfg.user == "jellyfin") {
       jellyfin = {
         group = cfg.group;