Changes to Get gas excise duty rates

the response property data/items/oneOf[subschema #2]/tiers/items/toKwh became nullable for the status 200

response property data/items/oneOf[subschema #2]/tiers/items/toKwh list-of-types was narrowed by removing types null from media type application/json of response 200

added subschema #1 subschema #2 to the data/items response property oneOf list for the response status 200

added the new optional query request parameter date

the query request parameter year became optional

added the non-success response with the status 400

added the non-success response with the status 404

removed subschema #1 subschema #2 from the data/items response property oneOf list for the response status 200