Core: default data_version to 0 (#1668)

* Core: default data_version to 0

This allows new (ap-)worlds to function with old clients without having to define a version.

* Blasphemous: fix data_version

worlds/AutoWorld.py

@@ -159,7 +159,7 @@ class World(metaclass=AutoWorldRegister):
     location_name_groups: ClassVar[Dict[str, Set[str]]] = {}
     """maps location group names to sets of locations. Example: {"Sewer": {"Sewer Key Drop 1", "Sewer Key Drop 2"}}"""
 
-    data_version: ClassVar[int] = 1
+    data_version: ClassVar[int] = 0
     """
     Increment this every time something in your world's names/id mappings changes.