COMPAT: Default value of observed parameter in dissolve

[m-richards]

On pandas main, the default value of observed in df.groupby has changed from False to True (pandas-dev/pandas#57330) along with a deprecation period. In dissolve we should probably make the same change, also with a deprecation period.

The future warning didn't show up in our tests because we supply observed explicitly as part of kwarg forwaring into two calls to groupby, one for the geometry and the other for the attributes.