South Carolina sales tax cut for people age 85 and older

Fixes PolicyEngine#5387

changelog_entry.yaml

@@ -0,0 +1,4 @@
+- bump: minor
+  changes:
+    added:
+    - South Carolina sales tax cut for people age 85 and older.

policyengine_us/parameters/gov/states/sc/tax/sales/exclusion/age_threshold.yaml

@@ -0,0 +1,10 @@
+description: South Carolina provides a lower state tax rate for sales and use by individuals of this age and older.
+values:
+  2021-01-01: 85
+
+metadata:
+  unit: year
+  label: South Carolina sales tax exclusion for the elderly
+  reference: 
+    - title: SC REVENUE RULING 08-5
+      href: https://dor.sc.gov/resources-site/lawandpolicy/Advisory%20Opinions/RR08-5.pdf#page=2

policyengine_us/parameters/gov/states/sc/tax/sales/exclusion/percentage.yaml

@@ -0,0 +1,10 @@
+description: South Carolina provides the following percentage of exclusion for sales and use tax.
+metadata:
+  unit: /1
+  label: South Carolina senior exemption amount
+  reference: 
+    - title: SC REVENUE RULING 08-5
+      href: https://dor.sc.gov/resources-site/lawandpolicy/Advisory%20Opinions/RR08-5.pdf#page=2
+
+values:
+  2021-01-01: 0.01