Set version to 1.6.0-async-await.1

Sources/GRPC/Version.swift

@@ -25,5 +25,5 @@ internal enum Version {
   internal static let patch = 0
 
   /// The version string.
-  internal static let versionString = "\(major).\(minor).\(patch)-async-await.2"
+  internal static let versionString = "\(major).\(minor).\(patch)-async-await.1"
 }