Update VAT reform impact expectation (#1540)

Author: MaxGhenis

policyengine_uk/tests/microsimulation/reforms_config.yaml

@@ -25,7 +25,7 @@ reforms:
   parameters:
     gov.hmrc.national_insurance.class_1.rates.employee.main: 0.1
 - name: Raise VAT standard rate by 2pp
-  expected_impact: 18.9
+  expected_impact: 21.3
   tolerance: 1.5
   parameters:
     gov.hmrc.vat.standard_rate: 0.22