fix: List executions are performed in parallel

Previously the `await group` was inside the for loop, effectively making it an async queue, where operations were performed in serial.

Author: NeedleInAJayStack

Sources/GraphQL/Execution/Execute.swift

@@ -866,9 +866,9 @@ func completeListValue(
                 )
                 return (index, result)
             }
-            for try await result in group {
-                results[result.0] = result.1
-            }
+        }
+        for try await result in group {
+            results[result.0] = result.1
         }
         return results.compactMap { $0 }
     }