[Concurrency] Change availability for a TaskPriority extension

We need the `init?(JobPriority)` constructor to be
`StdlibDeploymentTarget 5.9`
so the compiler will not complain when we reference it
 from `ExecutorJob.createTrampoline` when building the standard library
without strict availability.

Addresses rdar://159397287

Author: edymtt

stdlib/public/Concurrency/PartialAsyncTask.swift

@@ -601,12 +601,12 @@ public struct JobPriority: Sendable {
   }
 }
 
-@available(SwiftStdlib 5.9, *)
+@available(StdlibDeploymentTarget 5.9, *)
 extension TaskPriority {
   /// Convert this ``UnownedJob/Priority`` to a ``TaskPriority``.
   ///
   /// Most values are directly interchangeable, but this initializer reserves the right to fail for certain values.
-  @available(SwiftStdlib 5.9, *)
+  @available(StdlibDeploymentTarget 5.9, *)
   public init?(_ p: JobPriority) {
     guard p.rawValue != 0 else {
       // 0 is "undefined"