feat: deprecate calendar_multiget in favour of multiget

calendar_multiget() now emits a DeprecationWarning directing callers
to use multiget() instead.  Slated for removal in v4.0.

prompt: extract the deprecation warning from the multiget async-aware commit into a separate stalled PR for v4.0

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: tobixen

caldav/collection.py

@@ -1161,7 +1161,15 @@ def calendar_multiget(self, *largs, **kwargs):
         @author mtorange@gmail.com
         (refactored by Tobias)
         This is for backward compatibility.  It may be removed in a later release.
+
+        .. deprecated::
+            Use :meth:`multiget` instead.
         """
+        warnings.warn(
+            "calendar_multiget is deprecated, use multiget instead",
+            DeprecationWarning,
+            stacklevel=2,
+        )
         return list(self.multiget(*largs, **kwargs))
 
     def date_search(