Fix AsyncAwait build for mac and tv (#8449)

Author: paulb777

FirebaseStorageSwift/Sources/AsyncAwait.swift

@@ -15,7 +15,7 @@
 import FirebaseStorage
 
 #if swift(>=5.5)
-  @available(iOS 15, *)
+  @available(iOS 15, tvOS 15, macOS 12, *)
   public extension StorageReference {
     /// Asynchronously downloads the object at the StorageReference to a Data object in memory.
     /// A Data object of the provided max size will be allocated, so ensure that the device has