Drop unsupported clients if they try to sync a v2 collection

ankisyncd/sync_app.py

@@ -82,6 +82,8 @@ class SyncCollectionHandler(anki.sync.Syncer):
             return Response(status=501)  # client needs upgrade
         if v > SYNC_VER:
             return {"cont": False, "msg": "Your client is using unsupported sync protocol ({}, supported version: {})".format(v, SYNC_VER)}
+        if v < 9 and self.col.schedVer() >= 2:
+            return {"cont": False, "msg": "Your client doesn't support the v{} scheduler.".format(self.col.schedVer())}
 
         # Make sure the media database is open!
         if self.col.media.db is None: