nixos: services: servarr: migrate prowlarr

It's configured in the exact same way as other *arr apps, and thus
automatically gains the declarative port configuration now.

I also want to move the other two indexer modules under `servarr`, as
they are all closely related.

modules/nixos/services/servarr/starr.nix

@@ -5,6 +5,7 @@ let
   cfg = config.my.services.servarr.${starr};
   ports = {
     lidarr = 8686;
+    prowlarr = 9696;
     radarr = 7878;
     readarr = 8787;
     sonarr = 8989;