Parameterize 28% on line 41 of AMT form

[MaxGhenis]

We currently hard-code this:

policyengine-us/policyengine_us/variables/gov/irs/tax/federal_income/alternative_minimum_tax.py

Line 82 in c6ecdc2

dwks41 = 0.28 * dwks40

See https://www.irs.gov/pub/irs-pdf/f6251.pdf (now line 39 instead of 41)

I think this is represented in https://www.law.cornell.edu/uscode/text/26/55#b_1_A

The tentative minimum tax for the taxable year is the sum of—
(i)26 percent of so much of the taxable excess as does not exceed $175,000, plus
(ii)28 percent of so much of the taxable excess as exceeds $175,000.
The amount determined under the preceding sentence shall be reduced by the alternative minimum tax foreign tax credit for the taxable year.

If I understand it correctly, this is already parameterized in https://github.com/PolicyEngine/policyengine-us/blob/master/policyengine_us/parameters/gov/irs/income/amt/brackets.yaml, as the 2% extra beyond the 26% base rate. So we might not need to create a new parameter, though we may want to make this a marginal_rate scale parameter to better align with the law and form.

Note Tax-Calculator also hard-codes this, see https://github.com/PSLmodels/Tax-Calculator/blob/master/taxcalc/calcfunctions.py#L1806

[PavelMakarchuk]

This has been fixed in #4815