Fix Montana AGI additions/subtraction logic for social security benefits

[martinholmer]

In the 0.759.1 version, the MT addition and subtraction logic for social security benefits is incorrect.

The 2021 Form 2 (page 6) indicates that the amount of federally taxable social security benefits can be adjusted up or down when calculating MT AGI. Here is the form schedule for doing that:

.

But the PEUS code incorrectly adds all social security benefits to federal AGI when calculating MT AGI (see parameter file below). This is a particularly glaring error because, for many tax units, some social security benefits are already included in federal AGI. Also, the code does not include the possibility that there could be a social security benefits subtraction when calculating MT AGI.

policyengine-us/policyengine_us/parameters/gov/states/mt/tax/income/additions/additions.yaml

Lines 1 to 15 in 91f05ce

	description: Montana counts these sources as additions.
	values:
	2021-01-01:
	- social_security
	metadata:
	unit: list
	period: year
	label: Montana adjusted gross income additions
	reference:
	- title: 2021 Montana Form 2 | Montana Subtractions Schedule
	href: https://mtrevenue.gov/wp-content/uploads/mdocs/form%202%202021%20instructions.pdf#page=21
	- title: 2022 Montana Form 2 | Montana Subtractions Schedule
	href: https://mtrevenue.gov/wp-content/uploads/dlm_uploads/2022/12/Form-2-2022-Instructions.pdf#page=22
	- title: Administrative Rules of Montana | Income Taxes | 42.15.2:Montana Additions and Subtractions
	href: https://rules.mt.gov/gateway/Subchapterhome.asp?scn=42%2E15%2E2