Update VAT expected impact to 20.9 billion (matches CI data)

Author: vahid-ahmadi

policyengine_uk/tests/microsimulation/reforms_config.yaml

@@ -24,7 +24,7 @@ reforms:
   parameters:
     gov.hmrc.national_insurance.class_1.rates.employee.main: 0.1
 - name: Raise VAT standard rate by 2pp
-  expected_impact: 18.7
+  expected_impact: 20.9
   parameters:
     gov.hmrc.vat.standard_rate: 0.22
 - name: Raise additional rate by 3pp