Move maximum_state_supplement and associated parameters to state folders

We currently model SSI state supplements in the `ssa` folder, but states write the rules around the maximum values. For consistency with state income taxes, we should move these parameters and logic into state folders.

I haven't looked at the legal code enough to know if we should also model `state_supplement` itself, which takes the maximum and phases it out given the federal SSI amount, at the state level. Since it's the same logic across states, leaving it as-is would be substantially simpler.