[Executors] Remove accidentally added availability on asUnownedSerialExecutor (#65407)

Author: ktoso

stdlib/public/Concurrency/Executor.swift

@@ -58,7 +58,6 @@ public protocol SerialExecutor: Executor {
 
   /// Convert this executor value to the optimized form of borrowed
   /// executor references.
-  @available(SwiftStdlib 5.9, *)
   func asUnownedSerialExecutor() -> UnownedSerialExecutor
 
   /// If this executor has complex equality semantics, and the runtime needs to compare