VED-1047: Update API service level on OAS (#1215)

* updated to Silver Plus

* kick

* remove long description

* changes as advised

* kick

Author: JamesW1-NHS

specification/immunisation-fhir-api.yaml

@@ -171,8 +171,7 @@ info:
      
     ## Service level
 
-    This API will be a platinum service, meaning it is operational and supported
-    24 x 7 x 365.
+    This API is a Silver Plus service.
      
     For more details, see [service
     levels](https://digital.nhs.uk/developer/guides-and-documentation/reference-guide#service-levels).