Update references with sources that actually corroborate 50% allocation and add 2024 null value

Author: MaxGhenis

policyengine_us/parameters/gov/states/mt/tax/income/deductions/itemized/spouse_allocation_rate.yaml

@@ -1,11 +1,12 @@
-description: Montana allocates itemized deductions to each spouse at this rate when filing separately.
+description: Montana divides itemized deductions equally between spouses when married filing separately on the same form.
 values:
   2022-01-01: 0.5
+  2024-01-01: null  # Tax simplification eliminated separate Montana itemized deduction calculation
 metadata:
   label: Montana itemized deductions spouse allocation rate
   unit: /1
   reference:
-    - title: Montana Form 2 Individual Income Tax Return 2022, Page 7 - Itemized deductions for separate filers
-      href: https://mtrevenue.gov/wp-content/uploads/dlm_uploads/2023/05/Montana-Idividiual-Income-Tax-Return-Form-2-2022v6.2.pdf#page=7
-    - title: Montana Code § 15-30-2131 - Deductions allowed in computing net income
-      href: https://law.justia.com/codes/montana/2022/title-15/chapter-30/part-21/section-15-30-2131/
+    - title: TaxSlayer Montana Itemized Deductions Guide - "divided equally between you and your spouse"
+      href: https://support.taxslayer.com/hc/en-us/articles/360015706992-What-type-of-manual-adjustments-do-I-need-to-make-to-my-Montana-Itemized-Deductions
+    - title: Montana Tax Simplification (2024) - Uses federal taxable income
+      href: https://revenue.mt.gov/montana-tax-simplification-resource-hub