fix: return obj unchanged if schema ver of instance if the target

The modified expression of
`schema_version == DANDI_SCHEMA_VERSION` is a
bug. Additionally, do comparison of versions
in tuples to a void missing cases of relation
between instance and target versions

dandischema/metadata.py

@@ -353,7 +353,7 @@ def migrate(
         schema = _get_schema(schema_version, "dandiset.json")
         _validate_obj_json(obj, schema)
 
-    if schema_version == DANDI_SCHEMA_VERSION:
+    if instance_ver == target_ver:
         return obj
 
     if version2tuple(schema_version) < version2tuple("0.6.0"):