Drop support for older versions of python-fsps

[prerakgarg07]

As of #196 the solar metallicity is taken from the stellar population object. This functionality was added to python-fsps in the commit dfm/python-fsps/pull/182 (August 11, 2022).

Use of older versions of python fsps with powderday has been deprecated and the support should be dropped altogether after the next public release of python-fsps