Fix kedro viz --lite compatibility with Kedro 18 (#2102)

This is to get Kedro-viz (mainly kedro viz --lite) to work with Kedro 18 and some Kedro 19 versions based on changes this PR (kedro-org/kedro#3920) introduced.

In July, Kedro made the _save and _load methods public. At that time, Kedro-viz did not rely on these methods. However, when we recently merged kedro viz --lite, we introduced an UnavailableDataset class, which is an AbstractDataset. This class now uses the public load and save methods. To maintain backward compatibility with older versions of the dataset, we followed a suggestion made by @deepyaman

class MyDataset(...):
    def _load(...) -> ...:
        ...

    load = _load

    def _save(...) -> ...:
        ...

    save = _save
Originally posted by @deepyaman in kedro-org/kedro#3920 (comment)

package/kedro_viz/integrations/utils.py

@@ -41,12 +41,15 @@ def __init__(
         self._data = data
         self.metadata = metadata
 
-    def load(self, *args, **kwargs):
+    def _load(self, *args, **kwargs):
         pass
 
-    def save(self, *args, **kwargs):
+    def _save(self, *args, **kwargs):
         pass
 
+    load = _load
+    save = _save
+
     def _exists(self):
         pass