CoW warning mode: enable chained assignment warning for DataFrame setitem in default mode
Follow-up on https://github.com/pandas-dev/pandas/pull/55522#discussion_r1406831714

This ensures the chained assignment warning is shown in the default mode as well. 
This does mean you currently get some noise cases of both this new warning and the existing SettingWithCopyWarning. But either this SettingWithCopy was a false positive and it was setting on a view (eg a far-fetched case like `df[0:3][0:2] = 10`), and then we really need to have the new warning as well because it will change behaviour. Or either it was a correct warning (e.g. `df[mask][other_indexer] = ..`, and in that case this has never been working, and adding an additional warning for a case that never works does no harm (and so a user can only have by accident, or potentially unaware it doesn't do what they intend).

xref https://github.com/pandas-dev/pandas/issues/56019
Thanks for the fixup