[Concurrency] SwiftStdlib 5.5 ⟹ SwiftStdlib 5.1 (usages)

The concurrency runtime now deploys back to macOS 10.15, iOS 13.0, watchOS 6.0, tvOS 13.0, which corresponds to the 5.1 release of the stdlib.

Adjust macro usages accordingly.

test/ModuleInterface/actor_old_compat.swift

@@ -10,7 +10,7 @@
 
 import OldActor
 
-@available(SwiftStdlib 5.5, *)
+@available(SwiftStdlib 5.1, *)
 extension Monk {
   public func test() async {
     method()