Update to pandas 2.0.0: remove .xls

[emlys]

pandas 2.0.0 was released yesterday and it's causing one HRA test to break. The error is

for filename, func in [('target.csv', source_df.to_csv),
                       ('target.xls', source_df.to_excel),
                       ('target.xlsx', source_df.to_excel)]:
    full_filepath = os.path.join(self.workspace_dir, filename)
>   func(full_filepath, index=False)
...
ValueError: No engine for filetype: 'xls'

I couldn't find this mentioned in the release notes, but it looks like they removed support for the .xls format (it's been deprecated for a while, and was provided by another library that's no longer maintained). We should accordingly remove the .xls test from HRA - if pandas isn't supporting it, I doubt there's much demand for this outdated format.

Also an opportunity to revisit whether we still want HRA to support excel files, since it's the only model that does so.