[Storage] Add preconcurrency attribute to class (#14272)

Author: ncooke3

FirebaseStorage/Sources/Internal/StorageInternalTask.swift

@@ -22,6 +22,7 @@ import Foundation
 
 /// Implement StorageTasks that are not directly exposed via the public API.
 @available(iOS 13, tvOS 13, macOS 10.15, macCatalyst 13, watchOS 7, *)
+@preconcurrency
 class StorageInternalTask: StorageTask {
   private var fetcher: GTMSessionFetcher?