Use prefixItems for tuples

+ From OpenApi 3.1
+ Preserves order for tuples
  + e.g., when generating typescript from generated openapi specs
    + (a, b) becomes [a, b] instead of [a || b, a || b]

src/Data/OpenApi/Internal.hs

@@ -1373,7 +1373,7 @@ instance ToJSON OpenApiItems where
     , "maxItems" .= (0 :: Int)
     , "example" .= Array mempty
     ]
-  toJSON (OpenApiItemsArray  x) = object [ "items" .= x ]
+  toJSON (OpenApiItemsArray  x) = object [ "prefixItems" .= x ]
 
 instance ToJSON Components where
   toJSON = sopSwaggerGenericToJSON