lowered min version for use of async-await (#29)

Author: mpoimer

Sources/Fetch/Extensions/Fetch+Async.swift

@@ -10,7 +10,7 @@
 
 import Foundation
 
-@available(macOS 12, iOS 13, tvOS 15, watchOS 8, *)
+@available(macOS 10.15, iOS 13, tvOS 13, watchOS 6, *)
 public extension Resource {
 
     enum ForwardBehaviour {