Switch to using an xfail rather than skip

Author: jsignell

tests/v1/test_item.py

@@ -688,7 +688,7 @@ def test_no_collection(item: Item) -> None:
     assert item.collection is None
 
 
-@pytest.mark.skip(reason="Still figuring out how to handle ext")
+@pytest.mark.xfail(reason=".ext is not supported for pystac v2")
 def test_migrate_by_default() -> None:
     with open(
         TestCases.get_path("data-files/projection/example-with-version-1.1.json")