Replace `employment_income` with `irs_employment_income` in `earned_income`

`earned_income` is used in IRS computations and should therefore be based on taxable wages and salaries, after pre-tax contributions.

The Maryland TANF also uses `earned_income`, so I think we should move that to use `employment_income` directly.

We may want to rename `earned_income` to `irs_earned_income` to clarify the consideration of pre-tax contributions.

I believe this is the final update to ensure that `employment_income` is only used for payroll taxes and benefits.